Allow readers to lock revisions

[Olly Betts]

If readers could lock the revision they were reading, DatabaseModifiedError could be avoided.

Brass now uses a freelist implementation to track the blocks which are free in each revision, so keeping multiple old revisions around is more feasible than it would be with the old base files.

Marking for 1.3.3 for now.

[Olly Betts]

In #40, the following approach is suggested:

[If] readers use fcntl locking on a single byte corresponding to the revision they're using (bytes off the end of the file can be locked, and shared locks on read-only files are ok). Then a writer would only delete old revisions for which it could obtain an exclusive lock (otherwise it would preserve them).

[Olly Betts]

Would be nice, but it's time to get brutal to get 1.4.0 out.

This could probably be done in a compatible way - a reader running an older version would just get DatabaseModifiedError (and probably a reader running a newer version could choose whether to lock or not, and if it didn't it would be just like a reader running an older version).