* Allow passing SYNAPSE_WORKER envvar
* changelog.d
* Document SYNAPSE_WORKER.
Attempting to imply that you don't need to change this default
unless you're in worker mode.
Also aware that there's a bigger problem of attempting to document
a complete working configuration of workers using docker, as we
currently only document to use `synctl` for worker mode, and synctl
doesn't work that way in docker.
