Make Database.releaseMemory() public

This allows us to document how to release memory asynchronously with a DatabaseQueue (in the next commit)

Author: groue

GRDB/Core/Database.swift

@@ -1184,7 +1184,8 @@ public final class Database: CustomStringConvertible, CustomDebugStringConvertib
 
     // MARK: - Memory Management
 
-    func releaseMemory() {
+    public func releaseMemory() {
+        SchedulingWatchdog.preconditionValidQueue(self)
         sqlite3_db_release_memory(sqliteConnection)
         schemaCache.clear()
         internalStatementCache.clear()