View Graphs and DatasetGraphMonitor - additions around DatasetGraphs

[Andy Seaborne <an...@apache.org>]

Some information on code that's just been checked in (rev 1466254).

The commit adds two pieces of machinery for DatasetGraphs.  Both are 
used by the text search module in /Experimental/jena-text but they are 
general pieces of functionality, so rather than leave in jena-text, they 
have gone into jena-arq in the hope of reuse.

Because both framework things were driven by one particular use case 
(jena-text) they may well need further changes to be more general.

== View Graphs

This is support for implementing the graph interface Graph over a 
DatasetGraph (either default graph or a named graph).

Normal in-memory datasets are the other way round - there graphs are the 
raw storage, you have a collection of graphs and want to have a 
dataset(graph).

jena-text works at the DatasetGraph level. The basic storage is quads or 
triples+quads and "view graphs" provides an implementation of Graph 
without needing additional adapter code.

TDB uses the same style but more slightly more efficiently.  It also 
need to expose the internal TDB tuple tables to the BGP evaluation 
(also, adds support for internal TDB checking).

== DatasetGraphMonitor
(AKA base-level code need for events for datasets graphs)

This is a wrapper that sends notification calls to a monitoring object. 
jena-text uses it to keep an index in-step with the dataset.

I have also a prototype that writes changes to a log, for either diff 
propagation or for replay-based transactionality for any DatasetGraph.

	Andy