Sherlock branch missing stat "ep_dcp_dead_conn_count"

Merging sherlock to watson triggered a failure as this stat goes away. Looking at the sherlock branch it appears a previous merge dropped the call to collect this stat from the connection map.

• 3.0.x has it https://github.com/couchbase/ep-engine/blob/3.0.x/src/ep_engine.cc#L3984
• sherlock it's gone https://github.com/couchbase/ep-engine/blob/sherlock/src/ep_engine.cc#L4084
• watson has it https://github.com/couchbase/ep-engine/blob/watson/src/ep_engine.cc#L4234

In watson it appears ep_testsuite is more sensitive to the stat going away, hence the sherlock->watson merge triggers a failure, yet sherlock doesn't fail.

ee1129699abfbbc4485f3c2ec2a9c4a53eb8a478 appears to have removed the stat call.