pgsql: Make pg_dump error cleanly with -j against hot standby

Make pg_dump error cleanly with -j against hot standby

Getting a synchronized snapshot is not supported on a hot standby node,
and is by default taken when using -j with multiple sessions. Trying to
do so still failed, but with a server error that would also go in the
log. Instead, proprely detect this case and give a better error message.

Branch
------
REL9_5_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/47e59697679a0877e0525c565b1be437487604a7

Modified Files
--------------
src/bin/pg_dump/pg_backup.h | 1 +
src/bin/pg_dump/pg_backup_db.c | 38 ++++++++++++++++++++++++++++++
src/bin/pg_dump/pg_backup_db.h | 1 +
src/bin/pg_dump/pg_dump.c | 53 +++++++++++-------------------------------
4 files changed, 53 insertions(+), 40 deletions(-)