refactor: split poll_transmit into per-path and per-space parts (#338)

* split up poll_transmit per path and per space

* refactor: return last_packet_number

* cleanup status enums

* refactor: streamline mtu probe building

* refactor: extract remote cid exhaustion handling into function

* refactor: move remote_cid check per path

* refactor: simplify send space check

* fixup

* docs(proto): describe enums

* wip

* friday was too short. make it work, maybe

* Do coalescing right

* tweak comments, logs

* remove redundant size-setting

* fix path scheduling description, add bug description

* fixup path stats

* wording, wording

* weird off path enum variants are no longer needed!

* fix fmt

* Make TransmitBuf in each loop and collate off-path work

This make the construction of the TransmitBuf a little less weird, but
still a bit weird.

* Consistent naming for now

* missing rename

* fix merge

* Pass around a Vec while no transmit has been started yet

* expand on what is meant here, link to new issue

* pointless return

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Co-authored-by: dignifiedquire <me@dignifiedquire.com>
Co-authored-by: Diva Martínez <26765164+divagant-martian@users.noreply.github.com>
Co-authored-by: Diva Martínez <git@divma.net>

Author: 3 people