Postgres master

**NOT INTENDED FOR MERGE YET, JUST DISCUSSION**

@jennyd and I are going to sit down and talk this through face-to-face, but other comments (particularly from @issyl0 and @fatbusinessman) should go here. We're aiming for merge tomorrow prior to postgres transition on Tuesday 28th.

We aren't turning synchronous commits in Production at the moment, so it could be misleading to have this here. We could set this only for the hits import in future, to try to improve performance for that.