Significant query throughput drop observed with v2.3.4

[Thejas-bhat]

We introduced changes that intend to publish IO stats (read/write) for tracking system usage with v2.3.4.

• A new stat for bytes read off the disk #1702
• Fix the IOStats computation #1710

For queries with high hit cardinality (order of 100K+ hits), we've observed a considerable query throughput drop in our in-house testing.

We are actively working on this issue to optimizing this stats calculation and perhaps even making this cost estimation on-demand to mitigate this regression.

The fix will be targeted for the v2.3.5 release.

[abhinavdangeti]

#1738