keeper: use and update lastPGState in keeper loop.
keeper: use and update lastPGState in keeper loop.

This avoids using an additional wal sender if the pg.GetPGstate() functions is
called in parallel with the pgState checker.

The pgStateChecker method has been renamed to updatePGState.

In the keeper loop, we must have an updated pgState, so we call updatePGState
when needed.

siepkes/stolon,siepkes/stolon,sgotti/stolon,sorintlab/stolon,sorintlab/stolon,smkingsoft/stolon,smkingsoft/stolon,sgotti/stolon