MM-11210 Add "GET /posts/unread" API to support landing on the last unread post (#11486)

* [MM-11210] Add API GET 'api/v4/channels/{channel_id:[A-Za-z0-9]+}/posts/unread' for scrolling overhaul (#9108)

* Add API GET 'api/v4/channels/{channel_id:[A-Za-z0-9]+}/posts/unread'

* add constants

* refactor GetPostSince and added more tests

* move constants to app package

* [MM-11528 &&  MM-11583] Add userId to in the "posts/unread" path and update test with time delay to fix intermittent failure (#9229)

* add userId to in the "posts/unread" path and update test with time delay to fix intermittent failure

* add limit before and after to query

* remove time delay on test and put pretermined value of Post.CreateAt

* Fix conflict

* [MM-11876] Add cursor to posts list such as next_post_id and previous_post_id (#9707)

* add cursor to posts list such as next_post_id and previous_post_id

add publish previous_post_id on WEBSOCKET_EVENT_POSTED and only get next or previous post IDs if necessary

revert change on adding previous_post_id in WEBSOCKET_EVENT_POSTED

add missing strings import

fix merge conflicts

* update per comment

* update per feedback

* corrected the logic in getting the next and previous post ID

* fix logic to determine next and post IDs, and rename function to have suffix of "Time"

* rearrange logics and add mote tests

* fix merge conflict

* fix missing message when using unread API (#10233)

* MM-15569 Fixes failing test on TestGetPostsForChannelAroundLastUnread (#11039)

* Fix missing posts when getting posts since

* revert changes to GetPostsSince

* migrate Post.GetPostAfterTime and Post.GetPostBeforeTime to sync by default

* revert change to cacheItem

* Fix post ID validation, build query on squirrel and only return post ID as necessary

store/sqlstore/post_store.go

@@ -4,6 +4,7 @@
 package sqlstore
 
 import (
+	"database/sql"
 	"fmt"
 	"net/http"
 	"regexp"
@@ -334,7 +335,7 @@ func (s *SqlPostStore) InvalidateLastPostTimeCache(channelId string) {
 
 func (s *SqlPostStore) GetEtag(channelId string, allowFromCache bool) string {
 	if allowFromCache {
-		if cacheItem, ok := s.lastPostTimeCache.Get(channelId); ok {
+		if cacheItem, ok := s.lastPostTimeCache.Get(channelId); ok && cacheItem.(int64) > 0 {
 			if s.metrics != nil {
 				s.metrics.IncrementMemCacheHitCounter("Last Post Time")
 			}
@@ -661,6 +662,78 @@ func (s *SqlPostStore) getPostsAround(channelId string, postId string, limit int
 	return list, nil
 }
 
+func (s *SqlPostStore) GetPostIdBeforeTime(channelId string, time int64) (string, *model.AppError) {
+	return s.getPostIdAroundTime(channelId, time, true)
+}
+
+func (s *SqlPostStore) GetPostIdAfterTime(channelId string, time int64) (string, *model.AppError) {
+	return s.getPostIdAroundTime(channelId, time, false)
+}
+
+func (s *SqlPostStore) getPostIdAroundTime(channelId string, time int64, before bool) (string, *model.AppError) {
+	var direction sq.Sqlizer
+	var sort string
+	if before {
+		direction = sq.Lt{"CreateAt": time}
+		sort = "DESC"
+	} else {
+		direction = sq.Gt{"CreateAt": time}
+		sort = "ASC"
+	}
+
+	query := s.getQueryBuilder().
+		Select("Id").
+		From("Posts").
+		Where(sq.And{
+			direction,
+			sq.Eq{"ChannelId": channelId},
+			sq.Eq{"DeleteAt": int(0)},
+		}).
+		OrderBy("CreateAt " + sort).
+		Limit(1)
+
+	queryString, args, err := query.ToSql()
+	if err != nil {
+		return "", model.NewAppError("SqlPostStore.getPostIdAroundTime", "store.sql_post.get_post_id_around.app_error", nil, err.Error(), http.StatusInternalServerError)
+	}
+
+	var postId string
+	if err := s.GetMaster().SelectOne(&postId, queryString, args...); err != nil {
+		if err != sql.ErrNoRows {
+			return "", model.NewAppError("SqlPostStore.getPostIdAroundTime", "store.sql_post.get_post_id_around.app_error", nil, "channelId="+channelId+err.Error(), http.StatusInternalServerError)
+		}
+	}
+
+	return postId, nil
+}
+
+func (s *SqlPostStore) GetPostAfterTime(channelId string, time int64) (*model.Post, *model.AppError) {
+	query := s.getQueryBuilder().
+		Select("*").
+		From("Posts").
+		Where(sq.And{
+			sq.Gt{"CreateAt": time},
+			sq.Eq{"ChannelId": channelId},
+			sq.Eq{"DeleteAt": int(0)},
+		}).
+		OrderBy("CreateAt ASC").
+		Limit(1)
+
+	queryString, args, err := query.ToSql()
+	if err != nil {
+		return nil, model.NewAppError("SqlPostStore.GetPostAfterTime", "store.sql_post.get_post_after_time.app_error", nil, err.Error(), http.StatusInternalServerError)
+	}
+
+	var post *model.Post
+	if err := s.GetMaster().SelectOne(&post, queryString, args...); err != nil {
+		if err != sql.ErrNoRows {
+			return nil, model.NewAppError("SqlPostStore.GetPostAfterTime", "store.sql_post.get_post_after_time.app_error", nil, "channelId="+channelId+err.Error(), http.StatusInternalServerError)
+		}
+	}
+
+	return post, nil
+}
+
 func (s *SqlPostStore) getRootPosts(channelId string, offset int, limit int) store.StoreChannel {
 	return store.Do(func(result *store.StoreResult) {
 		var posts []*model.Post