This commit introduces an api.Cid type and replaces the usage of cid.Cid
everywhere.
The main motivation here is to override MarshalJSON so that Cids are
JSON-ified as '"Qm...."' instead of '{ "/": "Qm....." }', as this "ipld"
representation of IDs is horrible to work with, and our APIs are not issuing
IPLD objects to start with.
Unfortunately, there is no way to do this cleanly, and the best way is to just
switch everything to our own type.
pinsvcapi: do not cache peer information here as all the needed information is
in the status objects.
This adds ipfs_addresses as a field broadcasted with the ping metrics.
