Allow user to force thread number

This allows the user to do (potentially) stupid things, but warns him.
The default is TBBs default, so probably the right thing.
To enforce thread numbers in extractor it must be passed to the child
threads.
Merge branch 'thread-control' of https://github.com/TheMarex/Project-OSRM into TheMarex-thread-control
Merge branch 'TheMarex-thread-control' into develop