mimic: rgw: civetweb: use poll instead of select while waiting on soc…

…kets

Non cherry-picked backport of 4d00358 as the
commit hashes of the submodules are different in different upstream release
branches of ceph/civetweb, so creating this new commit which directly references
the tip of ceph-mimic branch instead picking from ceph-master, as ceph-master
and ceph-mimic may diverge at a later stage.

Signed-off-by: Abhishek Lekshmanan <abhishek@suse.com>