Fix txcouchbase test (uri fixup)

Change-Id: I4060fff22c5941f46d656ed8f7a241872b0e7d77
Refactor basic KV api for lcb3 API

This makes the code smaller and clearer to read. WIP since we still need
to change the callback interfaces as well.

Change-Id: Ie8f6fcc1f2f922833db17a348bd12ad820ae9824