Threads: assigning IDs from hmap

Threads: assigning IDs from hmap

For certain upcoming data structures, we actually need to use thread IDs
as a functional information to index things, not just a logging token.
Thus, we need them to be dense, not just flying around as they were until now.

To achieve this, we assign the IDs from a global hmap when the threads
are started, and properly return them when the threads are finished.
This way, the IDs of stopping threads are expected to be recycled,
whereas until now it wasn't expected to happen.

You may need to take care about this in your log reading apparatus.

Also there is now a maximum thread count hard limit because unlimited
thread count is too crazy to handle. But the limit is still ridiculously
high and nobody is ever expected to hit it anyway.