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.

assigned to @Diziet

requested review from @nickm

Yes, TorClient needs access to the guardmgr so that it can call install_bridge_desc_provider.

lgtm, feel free to merge modulo my latest comment on !845 (merged)

merged

mentioned in commit b41becec