Simplify the ExecutionContext framework

The ExecutionContext interface is used throughout the system. It's defined in 'dna-graph', and used to represent the context or environment for executing various activities. There is also ExecutionContextFactory, which is an interface for creating other contexts with different JAAS security contexts (login context or access control contexts). There are also the Basic* implementations of these interfaces, as well as several subclasses (with various implementations).

The idea of using an interface for this was to simplify the design of the components that uses/took an ExecutionContext, to prevent propagating dependencies, and to allow customization. However, since ExecutionContext is largely just an aggregation of references, there's little need to customize the behavior, and this doesn't really add any other dependencies to the classpath (all implementations of the aggregated components are already in 'dna-graph') and it doesn't expose any implementations through the method signatures. Using interfaces also complicates the usage, since clients have to know about the concrete implementations. This is even more true when they're setting up the DNA services and repositories in 'dna-repository' and 'dna-jcr'.

So, consider changing this interface to a concrete class, with methods that make it easy to create instances as well as instances with custom components (e.g., a new context that is the same as 'this' but with a supplied NamespaceRegistry implementation).

This will change the public API, but this should be acceptable given that we're not yet at 1.0 (and this really improves both the client code, and doesn't really change extension implementations).