server: mono-process mode

The forking that the hgitaly-serve command is doing by default is harmful when we want to debug with pdb: request handling always end in an error. The new --mono-process option solves that by serving directly from the main process. This is probably a matter of forwarding all the right file descriptors for the tty, but it's not worth the investigation time when there is such a simple solution.

Could have done it in the command itself, but this is probably easier to maintain in the server module and certainly easier to test.

No strong assertions that it doesn't fork in the test, but coverage proves that the early return got executed; that should be enough.