Invalidating db-cache for select vertices only

I would like to request an API for selectively invalidating vertices in the database-level cache described in [11.3 Database-Level Caching](http://docs.janusgraph.org/latest/caching.html).

**Use Case**
When running multiple instances of JanusGraph on different servers, one may update a certain vertex and notify other servers of the update by vertex ID. If another server acts upon the update by e.g. pushing a change event to clients, it prepares a JSON representation of the vertex to transmit. This JSON representation is likely to contain stale data from the db-cache.

Any way to circumvent the cache and directly fetch data from the storage backend would be great, without having to disable the cache altogether.

**Example**
APIs like these would be handy:

`Vertex::invalidateCache()`
`JanusGraph::invalidateCache(Object... ids)`

Start with the Database-Level Caching section in the linked documentation and examine how the database-level cache is exposed. Define the scope of selective invalidation for vertex IDs, including the Vertex::invalidateCache() and JanusGraph::invalidateCache(Object... ids) examples. Done means callers can bypass or invalidate cached data for selected vertices without disabling the cache globally.

Written by the indexing model from the issue text.