Add anti-fee sniping when spending outputs

[tnull]

As of #105 we set the nLockTime field to 0 as we currently don't track the current block height yet and neither BDK's wallet nor lightning-transaction-sync allow us to get information about currently synced chain tip. (We should add this capability upstream).

Currently we could call ChannelManager::current_best_block then follow the recommendation of setting nLockTime to current height.

[tnull]

Nvm, just went ahead and did this in #105.