[Cbbackupmgr] Activity monitor not logging inactivity

Description:

The activity monitor does not appear to be logging inactivity.

Steps to reproduce:

1. Set CB_DCP_ACTIVITY_MONITOR_FREQUENCY equal to 5 seconds
2. Start a backup asynchronously.
3. When ' DCP ' is spotted in the logs repeat the following 5 times:
   1. pause the memcached process for a total of 7 seconds by sending a SIGSTOP.
   2. Unpause memcached by sending a SIGCONT.
4. Scan the logs for the inactivity message.

What happens:

In 6.6.1-9174, the inactivity message is not present in the backup logs. 

However, in 7.0.0-3583, the inactivity message is present:

2020-11-09T15:42:35.583+00:00 WARN: (DCP) (default) (vb 153) Stream has been inactive for 5s, last seqno 112 -- couchbase.(*DCPAsyncWorker).monitorActivity.func1() at dcp_async_worker.go:247

2020-11-09T15:42:35.583+00:00 WARN: (DCP) (default) (vb 167) Stream has been inactive for 5s, last seqno 113 -- couchbase.(*DCPAsyncWorker).monitorActivity.func1() at dcp_async_worker.go:247

2020-11-09T15:42:35.583+00:00 WARN: (DCP) (default) (vb 209) Stream has been inactive for 5s, last seqno 112 -- couchbase.(*DCPAsyncWorker).monitorActivity.func1() at dcp_async_worker.go:247

2020-11-09T15:42:35.583+00:00 WARN: (DCP) (default) (vb 223) Stream has been inactive for 5s, last seqno 112 -- couchbase.(*DCPAsyncWorker).monitorActivity.func1() at dcp_async_worker.go:247

What I expected to happen:

I expected the equivalent message to be present in the 6.6.1-9174. backup logs.

Logs for 7.0.0: 

cbbackupmgr-collectinfo-entbackup_10.112.210.101-2020-11-09T154345.zip

[^collectinfo-2020-11-09T154308-ns_1@cb.local.zip]

Logs for 6.6.1: 

cbbackupmgr-collectinfo-entbackup_10.112.202.101-2020-11-09T160246.zip

[^collectinfo-2020-11-09T160228-ns_1@cb.local.zip]