Comment 4 for bug 43731

I believe the agreed upon fix is to be able to supply additional transport objects to 'get_transport()' and ask it to re-use them if it can.

This needs to propagate up the stack a bit (so Branch.open_containing() might take a list of transports to re-use).

But this makes the 'cache' a very localized operation, and not global througout bzrlib.