MultiDirectorySession should be pool-aware or at least pool-enabled

A full-multi (SQL, LDAP) directories configuration has been setup and called with 300 users. We end up with 300 LDAP connections whether or not we set -Dcom.sun.jndi.ldap.connect.pool.maxsize=10 in $JAVA_OPTS in nuxeo.conf.

It would be expected to end up 300 sequential invocations with less than 300 LDAP connections and that LDAP pooling would work (The configuration <poolingEnabled> is enabled (true) by default for LDAP)

Debugging shows that MultiDirectorySession is instantiated upon every request making pooling ineffective in this setup at Nuxeo level.