MB-12271: Set the default dcp producer noop interval to 20 seconds

This is a fix to support backwards compatibility between 3.0 and
3.0.1+ versions of Couchbase. The problem is that 3.0 has a noop
interval of 20 seconds by default and 3.0.1 has a default noop
interval of 200 seconds. In 3.0.1 the consumer explicitly sets
the noop interval of the producer so in 3.0.1+ clusters the 20
second default will be overriden by the consumer, but in the
scenario when a 3.0.1+ node connects to a 3.0 node the 20 second
default will remain.

Change-Id: I2e18e9ad68037f3a82abe5167f2bca89f381f318
Reviewed-on: http://review.couchbase.org/42136
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>