lei q: support --jobs [SEARCHERS],[WRITERS]

lei q: support --jobs [SEARCHERS],[WRITERS]

This comma-delimited parameter allows controlling the number or
lei_xsearch and lei2mail worker processes.  With the change
to make IPC wq_* work use the event loop, it's now safe to
run fewer worker processes for searching with no risk of
deadlocks.

MAX_PER_HOST isn't configurable yet for remote hosts,
and maybe it shouldn't be due to potential for abuse.