lnrpc+rpcserver: add ConfirmationsUntilActive and ConfirmationHeight

Introduce ConfirmationsUntilActive and ConfirmationHeight in
PendingChannelsResponse_PendingChannel. ConfirmationsUntilActive 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). ConfirmationHeight records
the block height at which the funding transaction was first confirmed; if
unconfirmed, it will be 0.

Signed-off-by: Nishant Bansal <[email protected]>

Author: NishantBansal2003