Move guardmgr creation to arti-client, rather than within circmgr

Having this done within circmgr was irregular - most of our other key buildup functions are done in TorClient::create_inner. I'm about to move dirmgr::Store creation out of dirmgr's constructors, too.

This was prompted by trying to do #631 (closed), though it may not be strictly necessary for that any more.

The branch is on top of !845 (merged) which I think is unblocked, but has not yet been merged.