pgsql: Exit from base backups when shutdown is requested

Exit from base backups when shutdown is requested

When the exit waits until the whole backup completes, it may take
a very long time.

In passing, add back an error check in the main loop so we detect
clients that disconnect much earlier if the backup is large.

Branch
------
master

Details
-------
http://git.postgresql.org/gitweb?p=postgresql.git;a=commitdiff;h=688423d004f4092aed73c73a3281c281d476436d

Modified Files
--------------
src/backend/replication/basebackup.c |   14 +++++++++++++-
src/backend/replication/walsender.c  |   16 ++++++++--------
src/include/replication/walsender.h  |    2 ++
3 files changed, 23 insertions(+), 9 deletions(-)