main.go: reduce sigFig's value to make RAM usage be sane

[vponomaryov]

Some time ago (https://github.com/scylladb/scylla-bench/pull/75) the sigFig value was changed from 3 to 5 which greatly increased the RAM usage.

Comparison on the single machine and same command with '3' and '5' values for it.

Value is set to '3':

 time   ops/s  rows/s errors max    99.9th 99th   95th   90th   median mean
   1s   50000   50000      0 12ms   5.2ms  2.8ms  1.2ms  983µs  754µs  821µs
   2s   50100   50100      0 12ms   8ms    2.4ms  1.3ms  983µs  754µs  829µs
   3s   50245   50245      0 11ms   6ms    2.3ms  1.2ms  983µs  786µs  822µs
   4s   50062   50062      0 9.3ms  5.9ms  2.4ms  1.6ms  1ms    786µs  837µs
   5s   50059   50059      0 9.3ms  5ms    2ms    1.1ms  950µs  786µs  800µs
   6s   50032   50032      0 6ms    4.4ms  2.2ms  1.5ms  983µs  786µs  818µs
   7s   49954   49954      0 12ms   6.2ms  2.5ms  1.5ms  1ms    786µs  841µs
   8s   50245   50245      0 13ms   5.6ms  2.2ms  1.2ms  983µs  786µs  815µs

And for '5':

 time   ops/s  rows/s errors max    99.9th 99th   95th   90th   median mean
 1.1s   49972   49972      0 53ms   16ms   2.1ms  1.2ms  1ms    754µs  841µs
 2.1s   50778   50778      0 48ms   20ms   3.8ms  1.6ms  1.1ms  786µs  911µs
 3.3s   49400   49400      0 217ms  95ms   3.6ms  1.7ms  1.2ms  786µs  1.1ms
 4.2s   55178   55178      0 200ms  86ms   7.4ms  1.7ms  1.1ms  786µs  1.2ms
 5.2s   50070   50070      0 7.8ms  4.1ms  2.8ms  1.5ms  1ms    786µs  835µs
 6.3s   47207   47207      0 51ms   20ms   7.4ms  2ms    1.4ms  786µs  1ms
 7.3s   49845   49845      0 63ms   35ms   13ms   2.8ms  1.5ms  786µs  1.3ms  Killed

In the case of '5' it gets OOMKilled all the time.

So, set it to be '3' for now. Let's add an option in future if we need to redefine it.

[vponomaryov]

Closes this bug: https://github.com/scylladb/scylla-bench/issues/84

[dkropachev]

Thanks @vponomaryov , I've added such option - https://github.com/scylladb/scylla-bench/pull/87