Moved detection of conn errors to debugger.

Moved code for detecting special errors (ECONNRESET, ECONNREFUSED)
from session to debugger.

Modified debugger 'close' event to send close reason as the first parameter.
Removed unused code from session.js
Extracted frontend communication to FrontendClient

Moved methods like sendResponse, sendEvent, etc. to FrontendClient.
Modified Agents and FrontendCommandHandler to use this new object
instead of session.

Removed ping message since it is no longer needed (socket.io can keep
the connection alive or handle reconnect).
Extracted debugger communication to DebuggerClient

Moved debugger-related code from session to DebuggerClient.

Moved sendDebugRequest from debugger to DebuggerClient.

Fixed a bug where a front-end page served from an old node-inspector
process crashed the new node-inspector process after restart.
Started rename of Debugger.sendDebugRequest.

Moved sendDebugRequest code to a new method called 'request'.
Refactored 'break'/'exception' handling

Moved session methods for handling 'break' and 'exception' events
to a new class BreakEventHandler.
Moved session.attach to DebuggerAgent.

Modified DebuggerAgent to use DebuggerClient instead of session.

Moved initialization of debugging session from session to DebuggerAgent.
Cleanup: Agents depend on debuggerClient

Modified RuntimeAgent and PageAgent to depend on DebuggerClient instead of
session.

Removed DebuggerClient.sendDebugRequest method.
Moved afterCompile handler to ScriptManager.
Implemented DebuggerClient.clearBreakpoint
Changed fetchCallFrames to not fetch scope details

Modified CallFramesProvider so that it does not fetch scope details
together with call frames. It creates a special objectId instead.

RuntimeAgent.getProperties checks for this special objectId format
and delegates getProperties to CallFramesProvider, which sends
'scope' debugger request to get scope details.

This change should reduce the time between debugged process hitting
a breakpoint and node-inspector pausing in the browser.