Improve tombstone management

Addresses #82.  See also #102.

This PR is the result of two efforts, the second building upon the first:

1. A fix for all known merge mis-behavior and races with respect to tombstones and merging.  (This stopped roughly at the tip of the `slf-merge-panopticon` branch on 24 December.
2. Change the format of hint files (in a backward-compatible way) to make it possible to avoid storing tombstone'ed keys in the keydir.