Revert "Bring back Backuptransport"

This reverts commit 255a0e103b4e07180761a0fb6719b27eea97813c.
Revert "Handle backup transport registration dynamically"

This reverts commit cefba58d14f9669b57c16b4ea493779d882c43bd.
Revert "Harden against transiently unavailable backup transports"

This reverts commit 28b6d3829166935f50684c0152a154cd4f1bbb90.
Revert "Revert "Harden against transiently unavailable backup transports""

This reverts commit d0ce48a1b2ec6b4cbfdb80eb2f254eee0cc508a8.
Revert "Revert "Handle backup transport registration dynamically""

This reverts commit 15b64cdd0db26db737a975c4d6874356cab038af.