Fauxton: fix regression for document editor

Introduced with ebade0f2f82d4da60e386f4aeaf24ecb98181e81
Use `os:timestamp/0 instead of `erlang:now/0`

We don't need the guarantees of `erlang:now/0` in `make_cookie_time/0`,
so `os:timestamp/0` is just fine.
Remove unused variable and function call

Finally get rid of that compiler warning.
Send a real EventSource event for heartbeat

The EventSource connection can get stuck (in TCP half-open state*) and there's no way
for the client to detect that. This commit changes the way heartbeat is sent, instead of
sending a newline character, it sends an empty event of type heartbeat:

    event: heartbeat
    data:

This event doesn't have an id: field, so the client will retain its latest Last-Event-ID state.

This doesn't change the expectations of clients that used EventSource till now, because they
subscribe to the 'message' event type. To get the 'heartbeat' events a client will need to
explicitly subscribe to it:

    source.addEventListener('heartbeat', function () { /* cancel a timer that would otherwise reconnect the source */ });

* this can happen when you suspend your laptop, on flaky internet connection, ADSL reconnect,
bad wifi signals, bad routers etc. Pretty often in a typical internet usage nowadays.
Fauxton: fix loading of pouch

Added a red test which turned green