pgsql: More synchronous replication tweaks.

More synchronous replication tweaks.

SyncRepRequested() must check not only the value of the
synchronous_replication GUC but also whether max_wal_senders > 0.
Otherwise, we might end up waiting for sync rep even when there's no
possibility of a standby ever managing to connect.  There are some
existing cross-checks to prevent this, but they're not quite sufficient:
the user can start the server with max_wal_senders=0,
synchronous_standby_names='', and synchronous_replication=off and then
subsequent make synchronous_standby_names not empty using pg_ctl reload,
and then SET synchronous_standby=on, leading to an indefinite hang.

Along the way, rename the global variable for the synchronous_replication
GUC to match the name of the GUC itself, for clarity.

Report by Fujii Masao, though I didn't use his patch.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/b8bb8dbf20e13087f8e57ff4c42e81776ae5e545

Modified Files
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src/backend/replication/syncrep.c |    2 +-
src/backend/utils/misc/guc.c      |    2 +-
src/include/replication/syncrep.h |    5 +++--
3 files changed, 5 insertions(+), 4 deletions(-)