This commit makes the `ChanBuilder` type in `tor-chanmgr` usable by
consumers outside of that crate, like the doc comment for
`ChannelFactory` says you need to be able to do in order to turn your
`TransportHelper` into something useful.
As part of doing this, the `event_sender` its constructor takes needed
to be dealt with, since it was a crate-internal type that came from
inside the `ChanMgr`.
Enter `BootstrapReporter`: an opaque wrapper around that sender, now
provided as an additional argument to
`ChannelFactory::connect_via_transport`. You can now construct a
`ChanBuilder` outside this crate, and it'll still be able to report its
bootstrap status by unwrapping this new type that's threaded through
from the `ChanMgr`. (This was a fair deal of manually threading the type
through all the layers in this crate!)
Note that you cannot implement bootstrap updating using something that
isn't `ChanBuilder` yet due to the type being entirely opaque (but, of
course, we can figure out exactly what API the reporter should have
later, and add that capability in).
a3c9fc5b1a