Benchmarks for indexed feed replication

[staltz]

We need to measure how much time does it take for indexed feed of size 10k take. Based on that, calculate msgs/sec.

[staltz]

@arj03 Oh, this is done! https://github.com/ssbc/ssb-ebt/blob/master/benchmark/index-performance.js

[arj03]

Right, now I remember, because we already do the get check for index feeds we wanted to measure the performance.