Reduce seqindex delta writeamp (was: Investigate high writeamp in showfast tests)

In 1B read heavy test, writeamp seen is around 9.6. In theory we expect it to be around 4. 

Looking  at a single shard stats (file attached), it looks like lot of file count compactions are happening.
"NCompacts": 416,
"NFileCountCompacts": 300,

Further 73% of writes done are due to compactions:
"WriteAmp": 9.632818222045898
"BytesIncoming": 125205009,
"NWriteBytes": 1206077081,
"NWriteBytesCompact": 888291328,

Also it is odd that seqIndex L3 has high NWriteBytes as compared to total BytesIncoming seen by seqIndex.

"BytesIncoming": 126275385,
L3: "NWriteBytes": 206098432
 

YCSB A latency test also has very high write amp (7.5.0-3014)

90th Percentile READ Latency(ms), Workload A, 3 nodes, 1 bucket x 1.5B x 1KB, Uniform distribution, 30K ops/sec, 1% Resident Ratio, Magma

Again the reason seems to be NFileCountCompactions. Note from a pure write IOPS perspective, this is making us do more of them in comparison to Couchstore (65k iops vs 40k iops per node).

Note: We should check if this is seen in any other test as well.

10B write heavy test:
http://perf.jenkins.couchbase.com/job/rhea-5node2-10B/183/
http://cbmonitor.sc.couchbase.com/reports/html/?snapshot=rhea_760-1186_access_a805