Threaded CI engine

[Debilski]

Work in progress, but gets the job done already (needs better data structures for result passing and the counting may still be broken).

Run 100 matches (10 at the same time):

contrib/ci_engine.py run -n 100 -t 10

and then view the result:

contrib/ci_engine.py print-scores

As it turns out, the old approach of simply running a bunch of ci_engine processes in parallel on a server is not really accurate (sqlite gets overwritten unless it is explicitly locked, in which case it will just crash). We basically need one and only one thread to do all the db handling.

TODO:

• Better counting (new match selection)
• Elo is broken
• Hashing should be done concurrently
• Breaks when not hashed
• No detection when players have changed

[Debilski]

TODO: Split stdout/stderr per player. Add CLI command show-last-failed to print output for failed games

[Debilski]

Closes #826