Use non-deprecated libtorrent parameters and functions

Also added support for seed_mode
Replace deprecated session.num_connections with session_status.num_peers

This commit removes get_num_connections() from core and updates UIs
to use get_session_status with num_peers key.

Extra noise is from Flake8 changes
Put setup_translation back into start_daemon

A 3rd party plugin broke because using _() in core files so reverting
to prevent issues and may also be useful in future for translating
the help text.