Upgrading a read transaction to a write transaction will immediately
fail if another connection/process started a write transaction in the
meantime. For transactions that we expect to perform a write after a
read, starting with `BEGIN IMMEDIATE` gives us a write transaction
immediately instead of beginning with a read transaction and upgrading
it.
Change-Id: I82b8a3843382940a4fa4ffe45f30b365b690ff86
Lily Ballard
98ccb179f9