Allow to select the choking algorithm

libtorrent currently provides four different choking algorithms.
Allow to select the algorithm from the advanced settings.

See libtorrent manual for a brief description of each algorithm.

Issue #1938.
Allow to select the seed choking algorithm

libtorrent currently provides three different choking algorithms.
Allow to select the algorithm from the advanced settings.

See libtorrent manual for a brief description of each algorithm.

Issue #1938.