pgsql: Fix race condition if a file is removed while pg_basebackup is r

Fix race condition if a file is removed while pg_basebackup is running.

If a relation file was removed when the server-side counterpart of
pg_basebackup was just about to open it to send it to the client, you'd
get a "could not open file" error. Fix that.

Backpatch to 9.1, this goes back to when pg_basebackup was introduced.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/36e4456d78b6a27cdaf82280b98998fc3bf3e9c7

Modified Files
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src/backend/replication/basebackup.c |   41 +++++++++++++++++++++++++--------
1 files changed, 31 insertions(+), 10 deletions(-)