pgsql: Rethink checkpointer's fsync-request table representation.

Rethink checkpointer's fsync-request table representation.

Instead of having one hash table entry per relation/fork/segment, just have
one per relation, and use bitmapsets to represent which specific segments
need to be fsync'd. This eliminates the need to scan the whole hash table
to implement FORGET_RELATION_FSYNC, which fixes the O(N^2) behavior
recently demonstrated by Jeff Janes for cases involving lots of TRUNCATE or
DROP TABLE operations during a single checkpoint cycle. Per an idea from
Robert Haas.

(FORGET_DATABASE_FSYNC still sucks, but since dropping a database is a
pretty expensive operation anyway, we'll live with that.)

In passing, improve the delayed-unlink code: remove the pass over the list
in mdpreckpt, since it wasn't doing anything for us except supporting a
useless Assert in mdpostckpt, and fix mdpostckpt so that it will absorb
fsync requests every so often when clearing a large backlog of deletion
requests.

Branch
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REL9_2_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/e3981da37a2f071951e54d74d45ad47733cc74a6

Modified Files
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src/backend/storage/smgr/md.c | 437 ++++++++++++++++++++++++-----------------
1 files changed, 256 insertions(+), 181 deletions(-)