clock synchronization:

  * Made TimeSync more robust. Synchronization messages between DIR and TimeSync are now ignored if the round trip time is greater than MAX_RTT. No retries are performed.
servers:
  * reduced HearbeatThread log output in case of timeouts
  * set max. request timeouts on MRC and OSD to 15s in order to reduce timeout frequency

git-svn-id: https://xtreemfs.googlecode.com/svn/trunk@2841 f8528748-c2c0-11dd-ba5c-d3b7c2b3b534