cbbackupmgr backup throughput drop ~25% from mad-hatter GA build to 6.6.0 GA build

cbbackupmgr backup (sqlite) throughput seem to have dropped in 660 GA build compared to 650 & 651 GA builds 

Below are different set of cbbackupmgr backup tests with different bucket config & test variations (after leto client rebuild). Degradation seem to be consistent across all tests for sqlite .

http://showfast.sc.couchbase.com/#/timeline/Linux/tools/backup/SQLite

EE backup throughput (Avg. MB/sec), 4 nodes, 1 bucket x 100M x 1KB, Idle, Uncompressed

*leto cluster rebuild was done sept 26th . All the runs post leto rebuild are pretty consistent and reproducible .

**For now I have tagged this ticket to cheshire-cat even though degradation seem to gave introduced in 6.6.0 build .