pgsql: Mitigate "snapshot too old" performance regression on NUMA

Mitigate "snapshot too old" performance regression on NUMA

Limit maintenance of time to xid mapping to once per minute. At
least in the tested case this brings performance within 5% of when
the feature is off, compared to several times slower without this
patch.

While there, fix comments and whitespace.

Ants Aasma, with cosmetic adjustments suggested by Andres Freund
Reviewed by Kevin Grittner and Andres Freund

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/7e3da1c4737fd6582e12c80983987e4d2cbc1d17

Modified Files
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src/backend/utils/time/snapmgr.c | 85 +++++++++++++++++++++++++++++-----------
1 file changed, 62 insertions(+), 23 deletions(-)