wtwire: add CheckRemoteInit helper

[cfromknecht]

This PR is an extension of #2606 and #2605, adding a CheckRemoteInit helper method to the wtwire.Init message. Once the client is merged, this will checked in three different places with in the code base, so it makes sense to centralize this logic. We also add unit tests to ensure that the combination of features bits proposed via client and server will work together from the perspective of both the client and server.

Depends on:

• watchtower/wtwire/features: make wtwire features uniform #2605
• watchtower/multi: send connection features + chain hash in Init #2606

[Roasbeef]

Needs a rebase! Should be g2g afterwards.

[cfromknecht]

@Roasbeef rebased, ptal