support multiple couchbase configuration nodes

it is (technically) possible to have multiple configuration nodes (through CouchbaseClient(List< URI >...). this is important on restarting a tomcat while the main configuration node is down.

memcachedNodes="http://1.2.3.4:8091/pools http://1.2.3.5:8091/pools http://1.2.3.6:8091/pools"

on a couchbase installation there should be no backup of the session, since the replication is handled by the couchbase instances itself.