foundation: Fix problem of duplicate responseAvailable() calls.

Setting the listener of the RPCResponse was not part of the synchronized block. Consequently, the following race condition could occur:
- listener was set by registerListener()
- meanwhile, RPC Client called responseAvailable() on the set listener
- registerListener() noticed that request != null and called responseAvailable() on the RPCResponse

git-svn-id: https://xtreemfs.googlecode.com/svn/trunk@3353 f8528748-c2c0-11dd-ba5c-d3b7c2b3b534