R1:7387cd63acbe

Provide an "isolated" database connection for testing

Summary:
This provides a new connection which doesn't connect to
anything, so effects can be isolated to the current process (for
unit testing).

Test Plan:
Ran unit tests.

Reviewed By: aran
Reviewers: aran, tuomaspelkonen, jungejason
CC: aran, epriestley
Differential Revision: 193