Enumerate available tablespaces after starting the backup

This closes a race condition where if a tablespace was created
after the enumeration happened but before the do_pg_start_backup()
was called, the backup would be incomplete. Now that it's done
while we are in backup mode, WAL replay will recreate it during
restore.

Noted by Heikki.

Branch
------
master

Details
-------
http://git.postgresql.org/gitweb?p=postgresql.git;a=commitdiff;h=3866ff6149a3b072561e65b3f71f63498e77b6b2

Modified Files
--------------
src/backend/replication/basebackup.c |   77 +++++++++++++++++----------------
1 files changed, 40 insertions(+), 37 deletions(-)

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