pgsql: Reduce sinval synchronization overhead.

Reduce sinval synchronization overhead.

Testing shows that the overhead of acquiring and releasing
SInvalReadLock and msgNumLock on high-core count boxes can waste a lot
of CPU time and hurt performance.  This patch adds a per-backend flag
that allows us to skip all that locking in most cases.  Further
testing shows that this improves performance even when sinval traffic
is very high.

Patch by me.  Review and testing by Noah Misch.

Branch
------
master

Details
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http://git.postgresql.org/pg/commitdiff/b4fbe392f8ff6ff1a66b488eb7197eef9e1770a4

Modified Files
--------------
src/backend/storage/ipc/sinvaladt.c |   62 ++++++++++++++++++++++++++++++----
1 files changed, 54 insertions(+), 8 deletions(-)