tor-proto,tor-cell: Code refactoring and add support for sending XOFF messages

This MR adds support for sending XOFF messages when the amount of stream data bytes in the stream queue becomes too large.

The first few commits do some refactoring. The main changes are:

• removes the flowctl-cc feature of tor-proto
  • this makes the tor_proto::relaycell::flow_ctrl module stable, including the XON/XOFF cell types
• rename CtrlMsg::SendSendme to CtrlMsg::FlowCtrlUpdate and make it extensible
  • we'll use this in the future to send XON messages in addition to SENDME messages
• rename StreamSendFlowControl to StreamFlowControl
• combine some objects into a StreamComponents to try to make the code simpler

The code changes/additions are:

• change UnparsedRelayMsg::data_len to return a Result, and to return 0 if the cell is not a RELAY_DATA cell
  • there are two reasons for this:
    1. the max length allowed may be different for different cell versions, and there's no way for the caller to know what cell version the UnparsedRelayMsg came from
    2. by ensuring we only return a valid length, and we don't return a non-zero length for non-data cells, we hopefully prevent future bugs
  • there was some discussion related to this in !2976 (comment 3194933) (/cc @nickm)
• add wrappers around the stream mpsc channel to track how many stream data bytes are queued
• add support for sending XOFF messages

/cc @gabi-250 as there are circuit reactor changes here.