MM-14246 - Plugin framework: support transactional semantics with KV Store (#10634)

* MM-14246 - Plugin framework: support transactional semantics with KV Store

Rename old, new variable names

Moving New function to the bottom

* Made CompareAndUpdate sync, updated tests

* Removed going through channel in CompareAndSetPluginKey

* Inserting new key when oldValue is nil to KVCompareAndSet

* Updated error text to include CompareAndSet

plugin/api.go

@@ -459,6 +459,16 @@ type API interface {
 	// KVSet will store a key-value pair, unique per plugin.
 	KVSet(key string, value []byte) *model.AppError
 
+	// KVCompareAndSet will update a key-value pair,
+	// unique per plugin, to the given new value if the current value == the old value.
+	// Inserts a new key if oldValue == nil.
+	// Returns (false, err) if DB error occurred
+	// Returns (false, nil) if current value != old value or key already exists when inserting
+	// Returns (true, nil) if current value == old value or new key is inserted
+	//
+	// Minimum server version: 5.12
+	KVCompareAndSet(key string, oldValue, newValue []byte) (bool, *model.AppError)
+
 	// KVSet will store a key-value pair, unique per plugin with an expiry time
 	//
 	// Minimum server version: 5.6