Investigate session consistency scan latencies

In MB-46068, indexer code is enabled to use bucket seqnos by default for session consistent scans. This is done to ensure that indexer can support large concurrency in scans as the bottleneck was observed in fetching seqnos. from KV in earlier experiments.

In the latest set of experiments, there seems to be no improvement in session consistent scan latencies with bucket seqnos. Rather, there seems to be a regression in performance.

The goal of this ticket is to investigate if the scan latencies observed with bucket seqnos. for scans are expected or not. 

Link to runs:

http://perf.jenkins.couchbase.com/job/aether/1138/ -> 7.1.0 - 1805 -> 128 concurrency, Uses Collection seqnos, Latencies: 4.74, 6.36ms

http://perf.jenkins.couchbase.com/job/aether/1140/ -> 7.1.0 - 1805, 1024 concurrency, Uses Collection seqnos, Latencies: 28.48, 35.97ms

Toy build is based on 7.1.0-1805 with https://review.couchbase.org/c/indexing/+/166218

http://perf.jenkins.couchbase.com/job/aether/1146 -> Toy build, 128 concurrency, Uses Bucket seqnos, Latencies: 20.37, 26.4 ms

http://perf.jenkins.couchbase.com/job/aether/1144 -> Toy build, 1024 concurrency, Uses Bucket seqnos, Latencies: 43.69, 58.35ms