Replace TransportRegistry with AbstractPtMgr
It doesn't make much sense to have the pluggable transport manager be a registry, so replace its interface with a more narrowly defined, less generic version.
Other changes:
- instead of returning a &-reference, it returns an owned Arc, which should make the ptmgr easier to implement while allowing efficient reuse
- provision for error handling is added, but will probably be revised in a future commit pending discussion
- tor-ptmgr code that would generate warnings as a result of this change is temporarily removed
This is a split out version of !886 (merged), intended so work on #659 (closed) can proceed.