Update file nameservice format, use with IPFS conn

[bplatz]

IPFS connection types now by default use two nameservices - IPNS and filesystem. The filesystem nameservice is synchronous, and requires a successful write before a commit is confirmed. IPNS is asynchronous and will update as IPNS allows it to.

Note that any connection can have any number of nameservices configured and fed into the conn as an option - so the old behavior is still available if needed, just not the default.

At the same time, the file nameservice needed to be updated as it previously only wrote file system path names as the commit-address - but now it needs to be able to write IPFS commit addresses (and potentially others).

This PR not only makes this address update to the file nameservice, but also

• stores the nameservice file as JSON-LD which can contain additional metadata about the ledger and branches
• Starts to use the branch specification format, as previously discussed, as (<branch-name>) appended to the nameservice address. Because branching isn't fully built out, this is all hidden under the hood for now

Future steps:

• Cache the nameservice file so a read isn't required each time (likely with a TTL, and coupled with a notification system that invalidates cache on known updates)
• Implement a Fluree database nameservice - using the JSON-ld format, the nameservice files could easily be stored in a special nameservice fluree database
• Allow create API which doesn't create a DB (it wouldn't require a transaction), but reserves the namespace so a db can be placed there
• Have a status property on the JSON-LD, where actions such as reindexing could be noted, which would make a ledger temporarily unavailable

[bplatz]

Plan to close this PR without merging, instead favoring PR #758 - leaving open for now as we have at least one user currently relying on this branch.

[cap10morgan]

@bplatz Good to close this now?