Mark channel cooperative close requests as non-final sequence

[alexbosworth]

When closing cooperatively a cooperative close transaction is created (CreateCooperativeCloseTx
https://github.com/lightningnetwork/lnd/blob/master/lnwallet/channel.go#L6989)

The funding outpoint is added as an input to the transaction, using the final sequence number. Instead this should signal that the cooperative close is non-final to avoid a scenario where the cooperative close transaction is stuck in the mempool indefinitely

As a workaround, CPFP is usually an option available to either side to increase the fee rate of a stuck channel close. However since cooperative close output scripts may be arbitrary programs, a CSV locking condition may exist on the output to prevent CPFP from being an option.

In any case, it would allow for more flexibility and potential chain-savings to allow for easier RBF and since RBF is a simple policy rule and not a consensus rule, and any replacement transaction would require the cooperative signing of both sides, changing the sequence value would not change any security assumptions.

[Roasbeef]

FWIW, this would need a protocol change as right now it dictates a final sequence is used for co-op close.