pgsql: Avoid having autovacuum workers wait for relation locks.

Avoid having autovacuum workers wait for relation locks.

Waiting for relation locks can lead to starvation - it pins down an
autovacuum worker for as long as the lock is held. But if we're doing
an anti-wraparound vacuum, then we still wait; maintenance can no longer
be put off.

To assist with troubleshooting, if log_autovacuum_min_duration >= 0,
we log whenever an autovacuum or autoanalyze is skipped for this reason.

Per a gripe by Josh Berkus, and ensuing discussion.

Branch
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master

Details
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http://git.postgresql.org/gitweb?p=postgresql.git;a=commitdiff;h=32896c40ca766146312b28a5a0eb3f66ca0300ed

Modified Files
--------------
doc/src/sgml/config.sgml | 5 +++-
src/backend/commands/analyze.c | 15 ++++++++++++-
src/backend/commands/vacuum.c | 39 ++++++++++++++++++++++++++--------
src/backend/postmaster/autovacuum.c | 14 +++++++++--
src/include/nodes/parsenodes.h | 3 +-
5 files changed, 61 insertions(+), 15 deletions(-)