Each replica of maildir has:
  * Unique ID
  * Current version number
  * Sync version vector
  * For each message
    - id of last replica to modify tags
    - current version number of that replica when it modified tags
    - Set of tags as of current version number

Before syncing to or from another replica:
  * Bump current version number by one
  * Set own ID to new version number in sync version vector
  * Scan all messages.  If tag differs from last seen, set state to:
    - modifying replica := current replica's unique id
    - version number := current version number
    - set of tags := current tags in xapian database

To sync A -> B:
  B sends A its "sync version vector"
  A sends B its "sync version vector"
  A sends B message state messages more recent than A's sync vector
  For each such message:
    If B's message state is than A's sync vector:
      merge flags
      set modifying replica to B and version to B's current version
    Otherwise:  entirely replace B's state with A's state for this message
  B sets it's sync version vector to pairwise max of A's and B's.


Keeping local database up to date:
  Keep track of ctime of last scan
  Scan file system to find all files with more recent ctime
  Look up each file in xapian, check all filenames for document
      (because this could be a rename, in which case old name is gone)
  This handles both new files and renames, but not deletes
  Deletes require scanning whole database and calling access on files
    Deletes should be kept in database for a while to propagate them
    But shouldn't re-appear, old version on other replicas before sync time


* Xapian lets us iterate through:
    dirname, dir_docid
    docid, message-id
    dir_docid, filename, docid
    tag, docid

* Resolving a link conflict
    Let n_A be the number of links of a file in xxx/new on replica A
    Let c_A be the number of links in xxx/cur on replica A
    Let n_B and c_B be the same for replica B
    Put n links in xxx/new and c links in xxx/cur where
      - c = max(c_A, c_B)
      - n = max(n_A + c_A, n_B + c_B) - c