add a correct lock management for sessions: when a session is found in cache,...

add a correct lock management for sessions: when a session is found in cache, do not make lock calls to final session backend (MySQL for example) fix LEMONLDAP-742



git-svn-id: svn://svn.forge.objectweb.org/svnroot/lemonldap/branches/lemonldap-ng_version_1_4-bugfixes@3513 1dbb9719-a921-0410-b57f-c3a383c2c641