lnwallet: add PendingUpdates method to channel state machine
lnwallet: add PendingUpdates method to channel state machine

This commit adds a new method, “PendingUpdates” to the channel state
machine which is intended to be a source to give callers a hint as to
when an additional commitment signature should be sent independent of
any request/response book keeping.

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