Re: WIP: Failover Slots

Here's a new failover slots rev, addressing the issues Oleksii Kliukin
raised and adding a bunch of TAP tests.

In particular, for the checkpoint issue I landed up moving
CheckPointReplicationSlots to occur at the start of a checkpoint, before
writing WAL is prohibited. As the comments note it's just a convenient
place and time to do it anyway. That means it has to be called separately
at a restartpoint, but I don't think that's a biggie.

The tests for this took me quite a while, much (much) longer than the code
changes.

I split the patch up a bit more too so individual changes are more
logically grouped and clearer. I expect it'd be mostly or entirely squashed
for commit.