non blocking da v2: frame publisher wrapper

[samlaf]

wip exploration of the idea described in celo-org#213 (comment)
where I renamed DaPublisher to FramePublisher.

So far I'm not super convinced by this approach:

• I don't like the fact that FramePublisher wraps the txmgr, because it couples altda with ethda code, and will make understanding the logic harder for people who don't care about altda
• the fact that frames are put back in channels when eth txs fails (not sure why its done this way) makes this abstraction not encapsulating at all, since we still need to pass a channelManager to the FramePublisher

[samlaf]

Closing as no longer needed. #7 is the approach we are going for.