Limit checkpointer requests queue size

Limit checkpointer requests queue size

If the number of sync requests is big enough, the palloc() call in
AbsorbSyncRequests() will attempt to allocate more than 1 GB of memory,
resulting in failure.  This can lead to an infinite loop in the checkpointer
process, as it repeatedly fails to absorb the pending requests.

This commit limits the checkpointer requests queue size to 10M items. In
addition to preventing the palloc() failure, this change helps to avoid long
queue processing time.

Also, this commit is for backpathing only.  The master branch receives
a more invasive yet comprehensive fix for this problem.

Discussion: https://postgr.es/m/db4534f83a22a29ab5ee2566ad86ca92%40postgrespro.ru
Backpatch-through: 13