pgsql: Improve some check logic in pg_receivewal

Improve some check logic in pg_receivewal

The following things are improved:
- Fetch the system identifier from the source server before any
WAL streaming loop. This triggers extra checks to make sure that
pg_receivewal is still connected to a server with the same system ID
with a correct timeline.
- Switch umask() (for file creation mode mask) and RetrieveWalSegSize()
(to fetch the size of WAL segments) a bit later before the initial
stream attempt. If the connection was done with a database,
pg_receivewal would fail but those commands were still executed, which
was a waste. The slot creation and drop are now done before retrieving
the segment size.

Author: Bharath Rupireddy
Reviewed-by: Ronan Dunklau, Michael Paquier
Discussion: https://postgr.es/m/CALj2ACX00YYeyBfoi55Cy=NrP-FcfMgiYYx1qRUEib3yjCVoaA@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/499c9b1266395c5e4c22bd7b2cbdb7f5a64ea4fa

Modified Files
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src/bin/pg_basebackup/pg_receivewal.c | 33 ++++++++++++++++++---------------
1 file changed, 18 insertions(+), 15 deletions(-)