Use sync.Map for watching queries

[JyotinderSingh]

Refactor WatchList implementation using sync.Map for thread safety and performance improvements since we will read from this map more than we write to it (a client will likely subscribe to some queries once and then listen on them indefinitely)

• Replaced WatchList and WatchListMutex with sync.Map for concurrent access.
• Added AddWatcher and RemoveWatcher functions to manage watchers.
• Updated evalQWATCH to use the new AddWatcher function.
• Modified WatchKeys to utilize sync.Map methods for iterating and managing clients.
• Simplified regex matching and client notification logic within WatchKeys.