f0a2ee12

By: Tom Sydney Kerckhove <syd@cs-syd.eu>

Start the standby on demand

The standby was started whether or not anything read from it, so a suite
that never touches its replica paid for one anyway. replicatedDBStandby
is an IO Standby now, started by the first test that asks and by nothing
else, and the bracket became an acquire/release pair so that deferring it
was possible at all.

Worth about 0.17 seconds per process that never reads a replica, measured
after the base backup stopped waiting for a spread checkpoint. Before that
it would have been worth 13.5.

StandbySpec asks the primary what is streaming from it: nothing until
something asks for a standby, and the same standby however often it is
asked for after that.