Rework wait for AccessExclusiveLocks on Hot Standby

Simon Riggs <simon@2ndQuadrant.com>

Commit: 37c54863cf71a4a1126d21db8eb68974bef34374
Author: Simon Riggs <simon@2ndQuadrant.com>
Date: 2016-03-10T19:26:24Z
Releases: 9.6.0
Rework wait for AccessExclusiveLocks on Hot Standby

Earlier version committed in 9.0 caused spurious waits in some cases.
New infrastructure for lock waits in 9.3 used to correct and improve this.

Jeff Janes based upon a proposal by Simon Riggs, who also reviewed
Additional review comments from Amit Kapila

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