Added HeapProfiler._lookupHeapObjectId

Closes #473

Serializing and caching of logic moved from consoleAgent to injectorServer to be available from heapProfilerAgent.
We don't have other commands with `result.result`, the previous realisation is very speculative, I asked about this [here](https://github.com/node-inspector/node-inspector/pull/338#discussion_r16981579). The current view is more readable.