Added the ability to get the local port of a memcache_instance_st.

Added the ability to get the local port of a memcache_instance_st.
Sometimes when you're hunting bugs on a production webserver (with multiple workers) you want to tcpdump and see what's going on.
This patch adds the memcached_server_srcport method to the library which returns the local sourceport of the connection (if there's a connection).
We use it a lot.