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return a boolean instead of an error if an extention is found on a message

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

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Also remove deprecated selector interface which is out of date

testutils will be general, so move them

Add a ConnectTo method to the network layer to create connections

setup GraphSyncNetwork interfaces and provide an implementation based on libp2p.host

fix #1