lnrpc+rpcserver: add ConfirmationsUntilActive field

Introduce ConfirmationsUntilActive in
PendingChannelsResponse_PendingChannel. This field indicates the remaining
confirmations needed for the channel to become active. If the funding
transaction is unconfirmed, ConfirmationsUntilActive defaults to the total
required confirmations (NumConfsRequired).
Signed-off-by: Nishant Bansal <[email protected]>

Author: NishantBansal2003