In the next commit I change conf.settings to be initialized
at runtime instead of load time and so any import of
`from lbrynet.conf import settings` will be and stay None if
it happens before the initialization.
Put stream downloader options into its own class, and make stream
downloader options global to the stream type rather than specific
to each factory.
Show downloader options in the lbrynet-downloader-gui.
Make a class for downloader option choices, so that the descriptions
can be displayed.
In the console, if there are multiple choices for the download
option, make it a list selected by its index.
Make sure that the ConnectionManager closes properly when some of
the connections fail to open (e.g. due to a host being down)
