Automatically discover situations where HTMap should use "assume" delivery

[JoshKarpel]

It should be possible to detect some obvious situations where, without user config, we should set "assume" delivery. @tannenba suggests a simple check: if the filesystem and uid domains are the same as they are at the submit location across the entire pool, we should change delivery to "assume". We would perform this check during startup, possibly asynchronously.

HTMap should probably also emit some log messages, including warnings like "you are using Docker delivery, but there are no machines in your pool that can run Docker jobs".

This also implies that we should probably set get_env = True for "assume" delivery. Will need to confer with @johnkn on how this might effect the PATH on the execute machine in different situations.