ReequestManager makes requests

[hannahhoward]

Goals

Build a request manager that can send requests out to the network

Implementation

Similar to bitswap:

• Build a message queue to manage sending messages to a single peer
• Build a peer manager to track open connections, and manage delegating messages to the appropriate queue

Then:

• Add some test infrastructure for various types of failed selectors
• Add the RequestManager:
  • RequestManager receives a request, and returns a response channel and an error channel
  • RequestManager serializes request out to the network
  • ResponseCollector buffers responses and errors so RequestManager doesn't block even if the peer itself does not read from the channel
  • RequestManager processes errors with selectors specs and sends them out on error channel
  • RequestManager reads responses sent back and blocks, for now, and only temporarily, transmits all blocks to all peers
  • RequestManager transmits failure response codes from peers as errors
  • RequestManager closes response channel once a terminal response is received

NOT INCLUDED:

• any attempt to do any kind of selector traversal / verify (it's seperate tickets GraphSync user can validate a response from another node #9 & GraphSync user can receive a response from another node #8 - though some of 8 is covered here - i had to implement enough of GraphSync user can receive a response from another node #8 to test)

For Discussion

So, there's a wall of code in this PR and I could use suggestions on how to make it more easily reviewable

fix #5