Consider maturation period when channel funding UTXO is in a coinbase transaction

Currently, the channel manager assumes that as soon as a channel has six confirmations, its funding is locked. However, in the rare case that a channel gets funded from a coinbase transaction, those funds aren't spendable until the output matures 100 blocks later. We should add a flag to keep track of this edge case so we don't send the funding locked message prematurely.