Uw general improvements

This PR tries to address both issues #37 and #30.

Refactoring to ease tracing
The locks_server now keeps a central version counter for strictly increasing versions
A sliding-window message log, kept in the process dictionary, is used in locks_leader, to facilitate debugging should the election process hang.
An 'election vector' is used in locks_leader to better keep up with the status of the locking process, and leader_uncertain is triggered anytime the vectors don't match.

uwiger / locks