OSD-replication:

- implemented the background replication, so the OSDs could replicate the whole file without client-requests
- Now a RandomAccessFile-instance uses always the same replica. It only changes the default replica, if a replica is added/removed. Of Course it uses temporarily the other replicas, if the default one is not reachable.
- ReplicationStressTest is divided into different readers and one writer
- a new class for starting OSDs on a cluster and terminating them from time to time (simulating a breakdown)
- TransferStrategy uses given strategy from replication flags

git-svn-id: https://xtreemfs.googlecode.com/svn/trunk@789 f8528748-c2c0-11dd-ba5c-d3b7c2b3b534