Enabled serialized threading. Log tag cleanup.

Sqlite now runs in serialized threading mode. This
resolves any remaining threading problems, and
allows us to begin removing the locks from the
storage engine.

Cleans up the tag used in logger calls in Database.cs.
Add .walrus to .gitignore
Merge pull request #92 from couchbaselabs/serialized-threading

Enables sqlite's serialized threading mode.