pgsql: Log a better message when canceling autovacuum.

Log a better message when canceling autovacuum.

The old message was at DEBUG2, so typically it didn't show up in the
log at all. As a result, in most cases where autovacuum was canceled,
the only information that was logged was the table being vacuumed,
with no indication as to what problem caused the cancel. Crank up
the level to LOG and add some more details to assist with debugging.

Back-patch all the way, per discussion on pgsql-hackers.

Branch
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REL8_4_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/b09847e51438e5566a83d44546f8d84ce25e0bf4

Modified Files
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src/backend/storage/lmgr/proc.c | 25 +++++++++++++++++++++----
1 files changed, 21 insertions(+), 4 deletions(-)