multiple KV regressions on 6.5.0-2148

The 2148 code changes introduced several regressions across KV.
**

Changelog:
{{kv_engine changed from b269433c62a0c64ff681e1b6c268b123f8c75dbc to master }}
{{[+] bb20e26 MB-32117: Track disk-seqno in DurabilityMonitor }}
{{[+] fd53068 MB-32591: Introduce ChkPtQueueIterator }}
[+] c2abd60 MB-32484: Add CMake argument CB_VERBOSE_MEMCACHED_TESTAPP

Issues:

1) Low injection rate in DGM

Test: 
Avg. write queue (items), 4 nodes, 1B x 1KB, full ejection, ~200K inserts/sec, initial load
http://showfast.sc.couchbase.com/#/timeline/Linux/kv/dgm/all

2147 vs 2148:
http://cbmonitor.sc.couchbase.com/reports/html/?snapshot=hera_650-2147_load_c4bc&snapshot=hera_650-2148_load_41c0

Job and logs:
http://perf.jenkins.couchbase.com/job/hera/2259/console

2) Lower throughput with replicateTo=1

Test:
Avg Throughput (ops/sec), Workload A, 3 nodes, 12 vCPU, replicateTo=1
http://showfast.sc.couchbase.com/#/timeline/Linux/kv/ycsb/all

2147 vs 2148:
http://cbmonitor.sc.couchbase.com/reports/html/?snapshot=hebe_650-2148_access_9950&snapshot=hebe_650-2147_access_6f4a

Job and logs:
http://perf.jenkins.couchbase.com/view/Weekly/job/hebe/2872/console

3) Slow failover rebalance

Test:
Graceful failover (ms), 4 -> 3, 1 bucket x 100M x 2KB, 10K ops/sec
http://showfast.sc.couchbase.com/#/timeline/Linux/reb/failover/all

2147: 3 sec
2148: 13 sec

Job and logs:
http://perf.jenkins.couchbase.com/view/Weekly/job/hestia/3404/console