pgsql: Make logging about multixact wraparound protection less chatty.

Make logging about multixact wraparound protection less chatty.

The original messaging design, introduced in commit 068cfadf9, seems too
chatty now that some time has elapsed since the bug fix; most installations
will be in good shape and don't really need a reminder about this on every
postmaster start.

Hence, arrange to suppress the "wraparound protections are now enabled"
message during startup (specifically, during the TrimMultiXact() call).
The message will still appear if protection becomes effective at some
later point.

Discussion: https://postgr.es/m/17211.1489189214@sss.pgh.pa.us

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/5ed6fff6b729c3cce55d4abc8f695da93aa40a0d

Modified Files
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src/backend/access/transam/multixact.c | 21 +++++++++++++--------
src/backend/access/transam/xlog.c | 4 ++--
src/backend/commands/vacuum.c | 2 +-
src/include/access/multixact.h | 3 ++-
4 files changed, 18 insertions(+), 12 deletions(-)