Implement LockBufferForCleanup(), which will allow concurrent VACUUM

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

Commit: 55432fedd2b3383c0cd0724a70ad0ae5134710f3
Author: Tom Lane <tgl@sss.pgh.pa.us>
Date: 2001-07-06T21:04:26Z
Releases: 7.2.1
Implement LockBufferForCleanup(), which will allow concurrent VACUUM
to wait until it's safe to remove tuples and compact free space in a
shared buffer page.  Miscellaneous small code cleanups in bufmgr, too.

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