Bug 730387 - Add slapi_rwlock API and use POSIX rwlocks
    
    This adds a new slapi_rwlock API and uses it throughout the server.
    Internally, this API can allow either NSPR or POSIX rwlocks to be
    used for different platforms.  This patch makes it use the POSIX
    implementation on Linux platforms since the NSPR implementation
    does not safely allow re-entrant reader locks to be used.