[BUGFIX] Memcached backend not cleared across Cli/Web requests
This is because the backend is bound to the executing script + SAPI mode
and not the installation path + Flow context. This makes it
impossible to clear entries created in the Web with the
Cli cache flush commands, and vice versa.
Additionally the Flow context is not taking into account so
the Development/Production share the same cache, which can
lead to undesired behavior.
Releases: master, 2.1, 2.2, 2.3