pgsql: In basebackup.c, refactor to create verify_page_checksum.

In basebackup.c, refactor to create verify_page_checksum.

If checksum verification fails for a particular page, we reread the
page and try one more time. The code that does this somewhat complex
and difficult to follow. Move some of the logic into a new function
and rearrange the code a bit to try to make it clearer. This way,
we don't need the block_retry Boolean, a couple of other variables
move from sendFile() into the new function, and some code is now less
deeply indented.

Patch by me, reviewed by David Steele.

Discussion: http://postgr.es/m/CA+TgmoYt5jXH4U6cu1dm9Oe2FTn1aae6hBNhZzJJjyjbE_zYig@mail.gmail.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/053183138a7a11408d6faa9281001ff7b1ffee2e

Modified Files
--------------
src/backend/backup/basebackup.c | 188 ++++++++++++++++++++++------------------
1 file changed, 104 insertions(+), 84 deletions(-)