lib: add arrayu64 type

This is based largely on strarray, but with a few changes to make
more sense for dealing with numbers.

Also 'remove_all' returns the number removed.  Might port that to
strarray as well.
multi conversations: handle replica cleanup

Also - no more limits.  Also, yay.

1) always look at every CID on every record and choose the
   largest that matches your subject (or has no B record)

2) if you see a CID which has no 'B' record (we already read
   in the data for this, so it doesn't cost anything) and you
   find another CID with the right subject and a higher number,
   then delete that unused CID from every '<' record.

Point (1) avoids every creating a split conversation.

Point (2) cleans up remaining mess when the last rename replicates.