MM-21672: KVCompareAndSet improvements (#13858)

* allow ExpireInSeconds < 0

Allow `ExpireInSeconds < 0` for use with `KVSetWithOptions`. While this has no practical use in reality, it's much easier to thoroughly unit tests the underlying functionality if we can match the semantics of CompareAndSet. Strictly speaking, this is a breaking change, but not relative to the advertised semantics. Anyway, it's also not entirely unreasonable to treat a negative `ExpireInSeconds` as having already expired vs. marking it as never expired.

* updated tests, to break apart

* honour expiry in CompareAndSet

* honour expiry in CompareAndDelete

* honour expiry in List

* fail unique constraint exception for SaveOrUpdate

A unique constraint error on a `SaveOrUpdate` should not be ignored: we did not save or update the requested value, as someone else managed to write the record first.

Note this is handled differently in `CompareAndSet`, where we correctly swallow the error and return `false` to indicate we did not successfully save the value.

* unexport DEFAULT_PLUGIN_KEY_FETCH_LIMIT

* s/InternalServerError/BadRequest/ for failed SaveOrUpdate

Co-authored-by: mattermod <mattermod@users.noreply.github.com>

model/plugin_kvset_options.go

@@ -32,7 +32,7 @@ func (opt *PluginKVSetOptions) IsValid() *AppError {
 // NewPluginKeyValueFromOptions return a PluginKeyValue given a pluginID, a KV pair and options.
 func NewPluginKeyValueFromOptions(pluginId, key string, value []byte, opt PluginKVSetOptions) (*PluginKeyValue, *AppError) {
 	expireAt := int64(0)
-	if opt.ExpireInSeconds > 0 {
+	if opt.ExpireInSeconds != 0 {
 		expireAt = GetMillis() + (opt.ExpireInSeconds * 1000)
 	}