ETag support

[dantleech]

The JCR specification supports an mix:etag mixin.

3.7.12.1 mix:etag

[mix:etag] mixin

• jcr:etag (STRING) protected autocreated
  A jcr:etag property is an opaque string whose syntax is identical to that defined for entity tags in HTTP/1.1. Semantically, the jcr:etag is comparable to the HTTP/1.1 strong entity tag.
  On creation of a mix:etag node N, or assignment of mix:etag to N, the repository must create a jcr:etag property with an implementation determined value.

We should implement this in the NodeProcessor as a pluggable service with a default implementation.