This patch is pure black magic.

It takes an incomprehensible piece of code which juggles with
master-master state (vector clock), and turns it into another
hardly comprehensible piece.

However, all tests pass, so this alone must justify the changes.
Another reason is that they are made by my blessed hand,
thus destined to be better even than sliced bread.

On a more serious note it's an attempt to more carefully
control vclock state changes through a set of select calls:

- add a new server to vclock
- "follow" local clock with an update from a remote vclock;
supposedly this update is dealing with an authenticated
server, so less checks are necessary, and an error in vlcock
order is an indicator of a programming error or corruption,
so should lead to panic, not to an exception
- a call to get the next LSN for a local coordinate of vclock.

This patch also renames vclock.cc to vclock.c to make it
FFI friendly.
Merge branch 'master' of github.com:tarantool/tarantool