Previously, the default RestTemplate that is used OpenTsdbGaugeWriter
was not used with its default configuration. Notably this meant that
it would have infinite connect and read timeouts. This is problematic
as it can cause metric writing to hang and block the scheduler for
performing any other tasks.

This commit updates OpenTsdbGaugeWriter to use a default connect
timeout of 10 seconds and a default read timeout of 30 seconds. A
constructor has been added to ease the configuration of these
timeouts. The existing option of providing your own RestTemplate
(via setRestTemplate) remains.

Closes gh-4698