Refactor married keychains

* move handling of following keychains into the leading keychain
* move multisig threshold into the leading keychain
javadoc style nit: don't put text on the same line as a /** unless it all fits on a single line.

This javadoc isn't particularly useful: OK, it tells me what it's useful for, but not what that usage looks like. How am I supposed to use this API?