Debugger: Fix race conditions in block mode

Using waitForMessage() in the constructor after registerService() is
_not_ safe: You might get the first message already after the
registerService() and before the waitForMessage() call. Instead,
use QMutex/QWaitCondition to block the initialization. Also make
the use of the block mode explicit, since the service might already
be enabled also for non-blocking modes ...

Change-Id: I387bfe0627c80e2029acff71f86d12cd9ab58de1
Reviewed-by: Kai Koehne <kai.koehne@nokia.com>