[FTS] cbdatasource enable DCP NOOP's

Brought up by Manu, the DCP documentation recommends enabling NOOP's, which require additional control messages to be sent during the DCP connection setup/initialization handshake. See: https://github.com/couchbaselabs/dcp-documentation/blob/master/documentation/dead-connections.md

cbft uses the cbdatasource for DCP processing, which can already handle or process incoming NOOP msgs, but doesn't currently ask for or enable those NOOP's during the DCP connection initialization "control message" phase.

See here for an example of a control message...

https://github.com/couchbase/go-couchbase/blob/master/cbdatasource/cbdatasource.go#L1889

This ticket is to optionally add control message requests to cbdatasource to ask for NOOP's with a configurable interval.