Add a generic threads abstraction

This should make it easier to work with threads. It provides classes for mutexes
and condition variables. Additionally, there is a special CMutexGuard that
automatically unlocks the mutex on destruction and a CThreadPool class.

This thread pool is used to replace the thread pool in the sockets code.

Signed-off-by: Uli Schlachter <psychon@znc.in>
CJob: Add a way to do stuff on the main thread

This just moves the pipe from the socket code to the thread pool. However, now
all CJobs can use this and there is a single place for them to get deleted.

Signed-off-by: Uli Schlachter <psychon@znc.in>