Add support to lock manager for conditionally locking a lock (ie,

Tom Lane <tgl@sss.pgh.pa.us>

Commit: d8d9ed931e8a2370d3995c40af2eb3bda18aecb0
Author: Tom Lane <tgl@sss.pgh.pa.us>
Date: 2001-06-22T00:04:59Z
Releases: 7.2.1
Add support to lock manager for conditionally locking a lock (ie,
return without waiting if we can't get the lock immediately).
Not used yet, but will be needed for concurrent VACUUM.

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