Expose `GetConnection()` to subtypes

Given `GetConnection()` is the only way to get from the `SQLiteAsyncConnection` back to the `SQLiteConnectionWithLock`, it would be nice to mark it `protected` to enable any classes extending from `SQLiteAsyncConnection` to gain access to it in order to build on top of it.
Merge pull request #33 from jamesottaway/patch-1

Expose `GetConnection()` to subtypes