Implement block-based grace period for channel announcements

[Roasbeef]

Issue

Atm, as soon as a channel has reached the specified number of confirmations, then we immediately announce it to the network. Instead of the immediate announcement, we should be following the specification's prescription to wait until the funding transaction has reached at least 6 confirmations.

Steps to Completion

• Fixing this will require decoupling the waitForFundingConfrimation method from the announcement of the channels themselves. However, by announcement we only mean broadcasting the series of channel announcement messages to the greater network.

• As soon as the FundingLocked message is sent, we should still be immediately adding the new channel to our internal channel graph so it can become visible to the ChannelRouter during path finding.

[Crypt-iQ]

I can do this one. It is a standalone issue, right? As in it doesn't require #303 or #306 to be completed as well?

[Roasbeef]

@Crypt-iQ it doesn't require them, but makes sense to work on the others in tandem as they touch the same part of the codebase, and slight refactors in one will likely make it easier to do the rest.

[Roasbeef]

Closed by #411.