implement payment channel manager

[noot]

Issue summary

Task summary

as part of retrieval markets, the miner and the retriever need to be able to open a payment channel between them, so the retriever can incrementally pay the miner for some portion of the data.

within filecoin, there is already a payment channel actor that can be called into for this, however a manager around the actor is needed to allow users to easily open a channel, send payments on it, and close it.

Acceptance Criteria

• [ ] two users are able to open a payment channel between each other
• [ ] one user is able to incrementally pay the other over this channel
• [ ] the channel is closed when it's no longer needed

Other information and links

see https://github.com/filecoin-project/lotus/tree/master/paychmgr for the go implementation