client: Fixed missed execution of callbacks when the RPC client was shutdown.

In particular, the following situations were fixed:
- a request was accepted (sendRequest) but not assigned to any ClientConnection.
- a request was sent to the server, but no response was received yet.
- a request was sent to the RPC Client which was already shutdown.

Additionally, the following changes were made:
- Backported fix for handleTimeout() crashes from revision 2984.
- Minor fixes in accordance to the style guide.

On a side note:
These issues were discovered by enabling the Vivaldi thread. The thread did constantly fire requests since the sleep time was set to 0 seconds. This command was used:

valgrind --gen-suppressions=yes --leak-check=full --show-reachable=yes --vgdb=yes ./mount.xtreemfs demo.xtreemfs.org/demo ~/mnt -f --vivaldi-enable -d DEBUG --vivaldi-recalculation-interval 0 --vivaldi-recalculation-epsilon 0 --vivaldi-max-iterations-before-updating 1 --request-timeout 5

git-svn-id: https://xtreemfs.googlecode.com/svn/trunk@3032 f8528748-c2c0-11dd-ba5c-d3b7c2b3b534