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To reduce future import cycles, and expose more types properly, move the root package to an
interface/types file and move the main implementation into /impl. This also exposes an interface
other implementers can replace.

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

Move ResponseProgress to the types package and extract some testing functions to testutil

make sure order channels are consumed in repsonse collector doesn't block sends on the other

Make the request manager actually inject responses from the network, and perform a selector
verification

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

Handle failed request status codes from server and pass on errors

Cleanup request manager test to extract repeated code

Add a channel of asynchronous errors instead of only an initial synchronous one

Create a manager for outgoing requests