Send a real EventSource event for heartbeat

[gdamjan]

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.

[kxepal]

+1

[gdamjan]

I'm have doubts about the test, maybe it should check for heartbeat_count > 3 or something ?
also, will "make check" even run that? I don't see the link for browser tests in master

[kxepal]

@gdamjan with !!window.EventSource check it wouldn't be tested with make check since there is no any window (iirc, but you may always make sure about by making your test explicitly and always failed), but you may navigate to browser tests by direct url:
http://localhost:5984/_utils/couch_tests.html
While like was removed from sidebar, the page and test running suite is still available.

[gdamjan]

ok, thanks.
changes test passed in browser.

[KlausTrainer]

@gdamjan Aside from the typo it looks good overall. Would you mind fixing it so I can commit it?

[gdamjan]

done
https://git-wip-us.apache.org/repos/asf?p=couchdb.git;h=95600b7