pgsql: Fix leakage of cost_limit when multiple autovacuum workers are a

Fix leakage of cost_limit when multiple autovacuum workers are active.

When using default autovacuum_vac_cost_limit, autovac_balance_cost relied
on VacuumCostLimit to contain the correct global value ... but after the
first time through in a particular worker process, it didn't, because we'd
trashed it in previous iterations.  Depending on the state of other autovac
workers, this could result in a steady reduction of the effective
cost_limit setting as a particular worker processed more and more tables,
causing it to go slower and slower.  Spotted by Simon Poole (bug #5759).
Fix by saving and restoring the GUC variables in the loop in do_autovacuum.

In passing, improve a few comments.

Back-patch to 8.3 ... the cost rebalancing code has been buggy since it was
put in.

Branch
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REL8_4_STABLE

Details
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http://git.postgresql.org/gitweb?p=postgresql.git;a=commitdiff;h=fab2af30d6bb4ca893e2e9bf96863e6f1ce826d2

Modified Files
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src/backend/postmaster/autovacuum.c |   63 +++++++++++++++++++++++++----------
1 files changed, 45 insertions(+), 18 deletions(-)