R1:8a6e9194963f

Support DELETE queries in the isolated database connection

Summary: While we eventually need a plan to make this less of a toy black hole,
we can support DELETE now -- it's just SELECT that's tricky.

Test Plan: Ran unit tests.

Reviewers: zeeg, jungejason, btrahan

Reviewed By: btrahan

CC: aran, btrahan

Differential Revision: 1226