move TransferrerPool from assistant

move TransferrerPool from assistant

This old code will now be useful for git-annex beyond the assistant.

git-annex won't use the CheckTransferrer part, and won't run transferkeys
as a batch process, and will want withTransferrer to not shut down
transferkeys processes. Still, the rest of this is a good fit for what I
need now.

Also removed some dead code, and simplified a little bit.

This commit was sponsored by Mark Reidenbach on Patreon.