graph: refactor announcement handling logic

In this commit, we remove the `processUpdate` method which handles each
announement type (node, channel, channel update) in a separate switch
case. Each of these cases currently has a non-trivial amount of code.
This commit creates separate methods for each message type we want to
handle instead. This removes a level of indentation and will make things
easier to review when we start editing the code for each handler.

Author: ellemouton