channeldb: add InsertNextRevocation method to OpenChannel
channeldb: add InsertNextRevocation method to OpenChannel

This commit adds a new method to the OpenChannel struct:
InsertNextRevocation. This functionality is required in order to
properly implement the new funding workflow and commitment transaction
state machine.

In the prior state machine, we utilized the “initial revocation window”
which would provide both sides with the necessary revocation keys that
they needed to create new commitment states. The version of the
commitment state machine within the specification has been simplified a
bit, and instead essentially incorporates a revocation window of 1. The
window is initially filled at the _opening_ of the commitment
transaction, then never extended, only “slid down” from there on.

AndrewSamokhvalov/lnd,aakselrod/lnd,LightningNetwork/lnd,LightningNetwork/lnd,AndrewSamokhvalov/lnd,ddustin/lnd,ddustin/lnd,aakselrod/lnd