Implement event loop for command handling in SugarDB.

[kelvinmwinuka]

To simplify the implementation of the SugarDB engine, let's experiment with an event loop that handles requests and other events such as snapshots, expiring keys, etc.

The event loop will run in a single goroutine and will handle internal and TCP request events. The embedded API will bypass the event loop. Store access will be synchronized between embedded API calls and TCP events using a mutex.