Get db session from the context.

Get db session from the context.

The aim is to use a single sqlalchemy session for an RPC request.

The context object passed to EngineAPI methods is actually an RpcContext
which contains the same data as the RequestContext. The @request_context
decorator turns this back into a RequestContext which can now have other
behaviours added to it.

RequestContext now has a lazy loaded session attribute.

Save calls on created entities need to be passed the shared session.

Change-Id: Ied4e66deaca205362b84fb698f75cc872886607d