The transport layer was taught to optionally exchange the session
ID assigned by the trace2 subsystem during fetch/push transactions.
* js/trace2-session-id:
receive-pack: log received client session ID
send-pack: advertise session ID in capabilities
upload-pack, serve: log received client session ID
fetch-pack: advertise session ID in capabilities
transport: log received server session ID
serve: advertise session ID in v2 capabilities
receive-pack: advertise session ID in v0 capabilities
upload-pack: advertise session ID in v0 capabilities
trace2: add a public function for getting the SID
docs: new transfer.advertiseSID option
docs: new capability to advertise session IDs