Fix live preview refresh issues (again)

[jasonsanjose]

Replaces original pull #3852
#3761 I found 2 issues here:

• When hitting refresh rapidly, the inspector API was returning an error Inspected frame has gone. This was due to our keepAlive RemoteAgent function call trying to call to a page(frame?) that no longer exists. I've updated RemoteAgent to stop the keepAlive timer when a new page loads.
• After getting in the above error state where the inspector web socket is still open, there were async issues calling Inspector.connect() where the inspector may get a disconnect event in the middle of the connection process. This caused an error dialog to appear Unable to load live development page. I've changed connect() to wait for a new disconnect() promise to complete.
  Slow page reload (5s) if the main editor does not show the main file #3792 After switching the current live document, remove all HTTP request filters.

[jasonsanjose]

@redmunds ready to review again. I wasn't able to produce those console errors you saw from #3852. Could that be coming from an extension? The console log you pasted contains code that I couldn't find in Brackets core at all.

[redmunds]

This looks good. I cannot reproduce the exceptions, so not sure where they were coming from. Travis build failed, so I'll merge when that's resolved.

[jasonsanjose]

Hmm. @dangoor mentioned spurious jasmine-node errors earlier today. I can't reproduce these on my mac. Kevin, do you see any issues here? I think it's ok to merge.

[redmunds]

Merging.