Prevent tap notify thread from busy-looping.
Prevent tap notify thread from busy-looping.

In testing, adding this sleep made the server thousands of times
faster.

Change-Id: I1c56873a3d2167f7353a41fd7eafc753a1c669d7
Reviewed-on: http://review.northscale.com:8080/71
Reviewed-by: Trond Norbye <60edd2ef23891a753f231b0c6f161dc634079a93@gmail.com>
Tested-by: Dustin Sallings <1c08efb9b3965701be9d700d9a6f481f1ffec3ea@spy.net>

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