Implement prop359 relay cell format

This ticket is for implementing the new relay cell format of https://spec.torproject.org/proposals/359-cgo-redux.html#relay-cell-format , as adapted from proposal 340.

@jnewsome has done a lot of the abstraction work here, to make relay cell formats switchable.

Part of #1936 (closed).

Subtasks:

• Implement version-based encoding for relay messages. (So far, we have only version-based encoding.)
• Implement decoding for new cell format.
• Implement encoding for new cell format.
• Make Data message chunking depend on cell format.
• Remove all or most hardcoded references to RelayCellFormat::V0 in the codebase.