Improve uniqueness of connection_ids

[mdickinson]

Context: connection ids are integers used to match message senders with the paired message receivers for a given executor.

Currently the message routers use connection ids that are simple integers, and always start counting from zero.

To aid diagnosis in cases where things go wrong, it may make sense to use some kind of UUID instead. In theory this shouldn't be necessary, but we've seen some rather strange failure modes on Windows where it seems possible that distinct QObjects with non-overlapping lifetimes that happen to exist at the same memory address are being confused with each other at Qt / PyQt level.

[mdickinson]

but we've seen some rather strange failure modes on Windows

The explanations for these failure modes turned out to be rather more prosaic: tasks were being submitted from worker threads (see #305). However, more unique connection ids may still be a good idea.

[mdickinson]

Also, while we're here, "connection_id" is a strange name. "task_id" may be better.