Lock MDSQLiteConnection._updateBuffer before accessing it

[LucDeCaf]

Fixes #75.

The FlushUpdates method in MDSQLiteConnection enumerates the _updateBuffer property using LINQ, but this can fail if _updateBuffer is modified partway through the enumeration. It also calls _updateBuffer.Clear() at the end of the function, which means any updates triggered between _updateBuffer being enumerated and _updateBuffer being cleared are dropped.

This PR adds a lock around _updateBuffer and wraps all reads and writes of _updateBuffer in said lock. I considered using a ConcurrentQueue, but we would need a workaround for ConcurrentQueue.Clear() since that method doesn't exist in .NET Standard 2.0 (it would probably also be slower).