emacs: address: save hash

This allows the user to save the address hash so that it is much
faster for the first completion after a restart. This defaults to off
as there are privacy implications to saving this information.

The code tries hard to avoid overwriting the wrong file. It also notes
if changes have been made to any of the relevant user settings, so
that the user does not get surprising results (i.e., outdated options
being used). Finally it stores some version information so that is
easy for us to update the format of the save file.