MB-10860: retry checkpointing http requests on errors

Because we're using connection pool and because http server may always
close "used" connection without warning, we must expect errors on
attempts to reuse sockets.

Luckily our checkpointing POSTs are idempotent, so we can freely retry
them.

Change-Id: I8f487395eae25d1b806741250da859684fa0848a
Reviewed-on: http://review.couchbase.org/35746
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>