Imaplement blueprint 'memcached-separate-port'.

Blueprint 'Make memcached protocol an alternative, not a replacement':

Blueprint was implemented. Memcached now can work
in presence of RELOAD CONFIGURATION statement, even if it
changes a replica to master or a master to replica.

'memcached' configuration file option was removed.
Instead, two new optinos were added:
- memcached_port - optional, default 0, if set, specifies
which port speaks memcached protocol.
- memcached_expire, boolean, which defines whether or not
we run expire on memcached namespace. If not set,
we simply support the protocol, but ignore TTL and never
expire the keys.

Original memcached test was ported to tarantool testing environment.
cas test was disabled, because tarantool has a non-critical bug (in
concurrent environment cas operation can return 'SERVER_ERROR' instead of
'EXISTS')