matdbg: support multiple simultaneous clients.
matdbg: support multiple simultaneous clients.

Previously, if you accidentally had two matdbg tabs open in Chrome, the
web app would hang. This fixes two bugs that prevented multiple
simultaneous clients:

1) The websocket handler broadcasted edit events to only the most
   recent connection.

2) The number of server threads was only 2, but the actual web app
   requires at least 2 threads for each instance, due to the
   externally linked CSS file.

google/filament,google/filament,google/filament,google/filament,google/filament,google/filament,google/filament,google/filament,google/filament,google/filament,google/filament,google/filament