Using condition variables to wait for checkpoints

Hello hackers,

A user complained about CREATE DATABASE taking > 200ms even with fsync
set to off.  Andres pointed out that that'd be the clunky poll/sleep
loops in checkpointer.c.

Here's a draft patch to use condition variables instead.

Unpatched:

postgres=# checkpoint;
CHECKPOINT
Time: 101.848 ms

Patched:

postgres=# checkpoint;
CHECKPOINT
Time: 1.851 ms

-- 
Thomas Munro
https://enterprisedb.com