During bootstrap, expect server_uuid/cluster_uuid exist in the snapshot.

During bootstrap, work even if server_uuid/cluster_uuid are
present in the snapshot. This is necessary to be able to upgrade
bootstrap.snap over time.
As a proof of concept, rewrite bootstrap.snap from an output snap.

Begin counting lsns from 1, not from 2.

Fixes gh-352
Merge branch 'master' of https://github.com/tarantool/tarantool