Fix DebuggerAgent.setVariableValue to object

Fix a bug where object values { objectId: '<id>' } were not correctly
translated to V8 references { handle: <id>}.
RuntimeAgent: implement releaseObject() as no-op
Tighten findAllApplicationScripts heuristics

When the directory does not contain package.json, limit the traversal
to top level javascript files and well-known subdirectories:
    lib/
    node_modules/
    test/

This should prevent unintentionaly broad searches like
all files in C:\ drive.
Ignore EACCESS errors when listing sources
Cache results of ScriptFileStorage.listScripts.
Speed up initial parsing of source-map URLs

`ScriptManager._getSourceMapUrl` accepts an optional parameter
`scriptSource`.

`DebuggerAgent._reloadScript` asks V8 to include script sources
in the response.

This way it's not necessary to send another request for each script
that is loaded by V8 at the time when Node Inspector is attaching to
the process.