Provide a service to generate Sequences that are cluster safe

When a sequence number is needed and must be consistent across a cluster, the standard solution is to use a SQL Database.

We already have such a service in Nuxeo, but the implementation is far from ideal.

Audit Service requires to attach a sequence ID to the events :

• this sequence Id was previously generate by the DB in the JPA implementation
• we rely on this eventId for some part of the code (like Drive)

When moving the Audit Backend to ES, we need a way to maintain a cluster safe sequence id.

The idea is to implement a sequence generator service on top of ES using the approach described in :

http://blogs.perl.org/users/clinton_gormley/2011/10/elasticsearchsequence---a-blazing-fast-ticket-server.html