mu: cleanup server side; make sure not to loose 'personal' flag when
seeing same contact in non-personal context
mu4e: tweak the sorting algorithm a bit to take the personal flag into
account
which, if true, means that the contact was seen in a message where at least
one of the addresses in the recipients field was 'my' address (this is
decided when in mu-store-write.cc). using this, we can exclude mailing list
posts.