pgsql: Move synchronous_standbys_defined updates from WAL writer to BG

Move synchronous_standbys_defined updates from WAL writer to BG writer.

This is advantageous because the BG writer is alive until much later in
the shutdown sequence than WAL writer; we want to make sure that it's
possible to shut off synchronous replication during a smart shutdown,
else it might not be possible to complete the shutdown at all.

Per very reasonable gripes from Fujii Masao and Simon Riggs.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/727589995a3e027046f0da7af6ba665b0b47f89f

Modified Files
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src/backend/postmaster/bgwriter.c | 8 ++++++++
src/backend/postmaster/walwriter.c | 6 ------
src/backend/replication/syncrep.c | 6 +++---
3 files changed, 11 insertions(+), 9 deletions(-)