pgsql: Rework MultiXactId cache code

Rework MultiXactId cache code

The original performs too poorly; in some scenarios it shows way too
high while profiling. Try to make it a bit smarter to avoid excessive
cosst. In particular, make it have a maximum size, and have entries be
sorted in LRU order; once the max size is reached, evict the oldest
entry to avoid it from growing too large.

Per complaint from Andres Freund in connection with new tuple freezing
code.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/d881dd6233f4eec6404f003bb08312e9e650e0e2

Modified Files
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src/backend/access/transam/multixact.c | 52 ++++++++++++++++++++++++++------
1 file changed, 42 insertions(+), 10 deletions(-)