Send NOOP messages to the eBucketMigrator periodically.
Send NOOP messages to the eBucketMigrator periodically.

The upstream server should send TAP NOOP messages to
the eBucketMigrator periodically even in non-idle state because
each disk backfill task might take a long time in case of the long
waiting backfill dispatcher queue. Otherwise, the eBucketMigrator
will get timeout if it does not receive any messages from the
upstream server for 300 sec. Note that TAP NOOPs are just dummy
messages and consumed by the local eBucketMigrator only, but not
propagated to the downstream server. Therefore, this change won't
cause any bad overhead.

Change-Id: Idfe06579d29a1832b3779a636b8a30a64759d530
Reviewed-on: http://review.couchbase.org/8210
Tested-by: Chiyoung Seo <8b279095f11c26cc21045fda5a3e51debb005aa9@gmail.com>
Reviewed-by: Steve Yen <ef850f73f7832052d3c6d9b5ddd3a1716d9a57f1@gmail.com>

daverigby/ep-engine,daverigby/ep-engine,couchbase/ep-engine,sriganes/ep-engine,zbase/ep-engine,zbase/ep-engine,daverigby/kv_engine,sriganes/ep-engine,abhinavdangeti/ep-engine,couchbaselabs/ep-engine,hisundar/ep-engine,membase/ep-engine,jimwwalker/ep-engine,abhinavdangeti/ep-engine,owendCB/ep-engine,couchbaselabs/ep-engine,daverigby/ep-engine,couchbase/ep-engine,zbase/ep-engine,membase/ep-engine,abhinavdangeti/ep-engine,owendCB/ep-engine,teligent-ru/ep-engine,couchbase/ep-engine,membase/ep-engine,daverigby/ep-engine,owendCB/ep-engine,daverigby/kv_engine,couchbaselabs/ep-engine,couchbaselabs/ep-engine,daverigby/kv_engine,sriganes/ep-engine,daverigby/kv_engine,abhinavdangeti/ep-engine,owendCB/ep-engine,teligent-ru/ep-engine,couchbaselabs/ep-engine,zbase/ep-engine,sriganes/ep-engine,teligent-ru/ep-engine,couchbase/ep-engine,jimwwalker/ep-engine,jimwwalker/ep-engine,jimwwalker/ep-engine,hisundar/ep-engine,hisundar/ep-engine,teligent-ru/ep-engine,abhinavdangeti/ep-engine,membase/ep-engine,zbase/ep-engine,hisundar/ep-engine