Make memcached protocol an alternative, not a replacement

Currenlty, if you turn on memcached protocol support, you can't access Tarantool using the native protocol.
Make it an alternative, not a replacement of the binary protoocol.
Other namespaces can still be available using the binary protocol.
Besides, introduce a switch that turns off expiration -- this is a simple way to do memcachedb support.

Implementation details:
- remove 'memcached' option from the configuration file
- add 'memcached_port', which enables memcached protocol support if not 0, and memcached_expire, which enables
memcached expiration, if set.

Memcached will work correctly in local hot standby mode, in replica mode, and expiration thread will be stopped
when we go from master to replica, and started when we go back, from replica to master.

It turs out that memcached doesn't have notion of read-only mode right now, i.e. tries
to write to the server even if it is in read-only mode (replica), which can lead to very bad things (c).
This needs to be fixed along in this blueprint.