[WIP] Sequence Numbers powered Compare and Set operations

[bleskes]

Elasticsearch has long supported compare and set (a.k.a optimistic concurrency control) operations using internal document versioning. Sadly that approach is flawed and can sometime do the wrong thing. Here's the relevant excerpt from the resiliency status page:

When a primary has been partitioned away from the cluster there is a short period of time until it detects this. During that time it will continue indexing writes locally, thereby updating document versions. When it tries to replicate the operation, however, it will discover that it is partitioned away. It won’t acknowledge the write and will wait until the partition is resolved to negotiate with the master on how to proceed. The master will decide to either fail any replicas which failed to index the operations on the primary or tell the primary that it has to step down because a new primary has been chosen in the meantime. Since the old primary has already written documents, clients may already have read from the old primary before it shuts itself down. The version numbers of these reads may not be unique if the new primary has already accepted writes for the same document

This PR introduces a new sequence number based approach that doesn't suffer from this dirty reads problem. The purpose for the PR is get initial feedback and serve as a basis for a BWC discussion.

Concretely I would like to get feedback:

1. The Engine implementation
2. Rest API
3. I'm currently still using VersionConflictException, I'm thinking of renaming it to ElasticsearchConflictException
4. BWC aspects

Once we clear those out, I'll break this up into smaller PRs and add documentation

[bleskes]

This has now been fully implemented. Deprecation and removal of OCC via internal versioning will be tracked separately. Thanks all for the feedback.