dirmgr: Expopse DirMgrStore
This is an opaque type, and all you can do with it is make one, and pass it to the DirMgr
and BridgeDescMgr
constructors.
This is needed for #631 (closed). The solution is not the one mentioned in #631 (comment 2853665) and is in fact simpler than that.
The branch is on top of !845 (merged) which I think is unblocked, but has not yet been merged.