Avoid pin scan for replay of XLOG_BTREE_VACUUM in all cases
Replay of XLOG_BTREE_VACUUM during Hot Standby was previously thought to require complex interlocking that matched the requirements on the master. This required an O(N) operation that became a significant problem with large indexes, causing replication delays of seconds or in some cases minutes while the XLOG_BTREE_VACUUM was replayed. This commit skips the pin scan that was previously required, by observing in detail when and how it is safe to do so, with full documentation. The pin scan is skipped only in replay; the VACUUM code path on master is not touched here and WAL is identical. The current commit applies in all cases, effectively replacing commit 687f2cd7.
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