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Per https://github.com/ipld/specs/pull/205, metadata is now an extension, and further extensions
will be added soon

Adds callbacks to manage connections, also removes network receiver methods from public graphsync
interface, and updates libp2p to handle go-libp2p-core refactor

Updates IPLD Bridge to use latest go-ipld-prime and runs and actual round trip test using GraphSync,
go-ipld-prime, and real selectors

Convert GraphSyncRequest and GraphSyncResponse to structs, track by value, pass directly into add
methods on GraphSyncMessage

Create a manager for outgoing requests

Create a message queue to pool messages for a single peer and send them

Update to finalized protobuf format.
Also remove deprecated selector interface which is out of date

Add ToNet and FromNet functions that read/write to an io stream for use in network. Also type
response codes

Use a Cid as a root instead of a node, so that only one round trip is neccesary

Initial implementation of graph sync messages including protobuf struct and
serialization/deserialization

fix #2 fix #3