* op-batcher: Rewrite ChannelManager

The channel manager is now responsible for tracking in flight frames
and properly handling different error conditions.

* op-batcher: String() & TerminalString() for txID

* op-batcher: Remove channelPending bool

* op-batcher: Fix wraparound bug in timeout check

* op-batcher: Comments + Logging

* op-batcher: Properly exit loop