lib: Add per-message last modification tracking

lib: Add per-message last modification tracking

This adds a new document value that stores the revision of the last
modification to message metadata, where the revision number increases
monotonically with each database commit.

An alternative would be to store the wall-clock time of the last
modification of each message.  In principle this is simpler and has
the advantage that any process can determine the current timestamp
without support from libnotmuch.  However, even assuming a computer's
clock never goes backward and ignoring clock skew in networked
environments, this has a fatal flaw.  Xapian uses (optimistic)
snapshot isolation, which means reads can be concurrent with writes.
Given this, consider the following time line with a write and two read
transactions:

   write  |-X-A--------------|
   read 1       |---B---|
   read 2                      |---|

The write transaction modifies message X and records the wall-clock
time of the modification at A.  The writer hangs around for a while
and later commits its change.  Read 1 is concurrent with the write, so
it doesn't see the change to X.  It does some query and records the
wall-clock time of its results at B.  Transaction read 2 later starts
after the write commits and queries for changes since wall-clock time
B (say the reads are performing an incremental backup).  Even though
read 1 could not see the change to X, read 2 is told (correctly) that
X has not changed since B, the time of the last read.  In fact, X
changed before wall-clock time A, but the change was not visible until
*after* wall-clock time B, so read 2 misses the change to X.

This is tricky to solve in full-blown snapshot isolation, but because
Xapian serializes writes, we can use a simple, monotonically
increasing database revision number.  Furthermore, maintaining this
revision number requires no more IO than a wall-clock time solution
because Xapian already maintains statistics on the upper (and lower)
bound of each value stream.