Remove mojo::system::Channel::{Attach,Run}Endpoint().

Also, rework some tests work (since they were the only remaining users
of these methods). In particular, RemoteMessagePipeTest.Multiplex
becomes a bit more complicated. However, a side benefit is that the test
now reflects how multiple message pipes are actually established on a
channel (i.e., starting with the bootstrap message pipe, then "sending"
more message pipes over it).

R=darin@chromium.org

Review URL: https://codereview.chromium.org/673783007