mgm: FuseServer: Caps: move to unordered variants of the containers
This allows for a future change where we rely on hash tables which don't promise the ordering of keys like a std::map does.
Also GetCaps() returns a const. ref to the caps table rather than a mutable reference since the only user of the function seems to only require a view rather than a modify
Signed-off-by: Abhishek Lekshmanan abhishek.lekshmanan@cern.ch