docs: fixed description of top_n

...although it's still none-too-easy to understand what this feature
actually does.
config: added in more params seen in the wild
instrumentation: don't change middleware names

Downstream consumers will break if the middleware handler functions
change their name, because they introspect on the functions to find
specific middleware.
docs: tweaked for clarity
tracer: don't nest transactions, reuse them

Also started the process of moving the remaining tracer functionality
accessible through the agent where it belongs. This time:
Agent.prototype.getState -> Tracer.prototype.getState.
agent: minor cleanup and refactoring

delete has no place in V8 JavaScript.
docs: updated for new version
release: published 0.11.1