Create a sequence index

Problem/Motivation

As part of the Multiversion module in Contrib we have a sequence index. This index is incremented every time an entity is created or updated. It allows us to know what has changed between two points in the sequence. This is a must when performing a replication of content between two sites, or between workspaces on a single site.

Proposed resolution

Add a sequence index (this is in key value for Multiversion, but we could do it as a custom table).
Update the index every time content entities are created or updated.