[No review required] Take new cpp-common and libmemcached to fix https://github.com/Metaswitc...

...h/cpp-common/issues/9 and https://github.com/Metaswitch/cpp-common/issues/8

Fixes https://github.com/Metaswitch/cpp-common/issues/8.

Fixes https://github.com/Metaswitch/cpp-common/issues/9.