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

Non cherry-picked backport of 4d0035830e5783a828a275245b8bc3ae88edc417 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>
(cherry picked from commit aafffbfd92b45712232937045a5be7464376a060)

diff --git a/src/civetweb b/src/civetweb
index 60628927158634f92dd2fe1c20067563d1607502..ff2881e2cd5869a71ca91083bad5d12cccd22136 160000 (submodule)
--- a/src/civetweb
+++ b/src/civetweb
@@ -1 +1 @@
-Subproject commit 60628927158634f92dd2fe1c20067563d1607502
+Subproject commit ff2881e2cd5869a71ca91083bad5d12cccd22136