Memcached UDP

This patch fixes memcached support of udp protocol. Now UDP packets with wrong header can break  memcached server down. Also I've made some fixes in cleaning UDP sessions after closing.
Results of testing (UDP vs TCP):

UDP:
Results of memcached stress test:
Total number of connections: 10000
Number of seconds for test: 2.23
Number of successfull connections: 10000
Connections per second: 4494.25
TCP:
Results of memcached stress test:
Total number of connections: 10000
Number of seconds for test: 4.38
Number of successfull connections: 10000
Connections per second: 2280.87

patch-memcached-udp