…ocs.

* Fix a bug that caused the ping interval to be less frequent than
  configured.
* Fix erroneous documentation of the websocket_ping_timeout default and
  clarify units for the ping interval.

Includes end-to-end tests that the correct number of pings are sent
(piggybacking on an existing test) and a unit test for the
`ping_sleep_time` calculation.

websocket_ping: fix ping interval with non-zero timeout and improve docs

Found thanks to #3510

Add overloads to gen.multi(). Closes #3514.

This argument was mistakenly removed in Tornado 6.5.0 with no warning;
it is now back but deprecated. The host header should be used instead.

Updates #3468

This was previously being logged as an uncaught exception in application
code, which is wrong for a malformed request. HTTPInputError now passes
through the app-error logging to be caught and reported as a 400
(which logs at the warning level to the access log and info to the
general log).

Fixes #3510

Backport bug fixes to 6.5 branch for 6.5.2

Release notes for 6.5.2