MM-59933: Increment/Decrement the member count directly (#28196)

While using Redis, there is no need to invalidate
and then update the new count from the DB when
we know the exact number it is going to be incremented
or decremented by.

In that case, we can directly use Redis primitives
to update the cache and prevent yet another DB query.

To achieve this, we modify the LRU cache get/set
paths slightly to not marshal into byte slices
for *int64 values. This is needed for Redis to operate
the INCR/DECR commands.

https://mattermost.atlassian.net/browse/MM-59933

```release-note
NONE
```

Co-authored-by: Mattermost Build <build@mattermost.com>

server/platform/services/cache/cache.go

@@ -52,3 +52,17 @@ type Cache interface {
 	// Name returns the name of the cache
 	Name() string
 }
+
+// ExternalCache is a super-set of the Cache interface with
+// a couple of more methods that allows for more efficient cache updates.
+// This can be achieved because the cache is external and an update
+// is visible to all nodes.
+type ExternalCache interface {
+	Cache
+	// Increment will increment the
+	// number stored at that key by the value.
+	Increment(key string, val int) error
+	// Decrement will decrement the
+	// number stored at that key by the value.
+	Decrement(key string, val int) error
+}