Implement log function natively

[zlin179]

Describe your changes
Add the log function similar to Graphite's log

Testing performed

• Unit Test (pass)
• Integration Test (pass)

I tested using mtcmptest with the following json

{
    "log()": "log(seriesByTag('name=jitter.g'))",
    "log(0.8)": "log(seriesByTag('name=jitter.g'),0.8)",
    "log(2.73)": "log(seriesByTag('name=jitter.g'),2.73)",
    "log(10)": "log(seriesByTag('name=jitter.g'),10)",
    "log(233333333)": "log(seriesByTag('name=jitter.g'),233333333)"
}

Basically they are the same except that the tag for log is {string: string} in mt & {string: float} in graphite, and very occasionally there's 1 bit precision difference.
link to the comparison result

Additional context
Benchmark

goos: darwin
goarch: amd64
pkg: github.com/grafana/metrictank/expr
cpu: Intel(R) Core(TM) i7-9750H CPU @ 2.60GHz
BenchmarkLog10k_1NoNulls
BenchmarkLog10k_1NoNulls-12                        39400             28115 ns/op
BenchmarkLog10k_10NoNulls
BenchmarkLog10k_10NoNulls-12                        4143            300366 ns/op
BenchmarkLog10k_100NoNulls
BenchmarkLog10k_100NoNulls-12                        342           3290549 ns/op
BenchmarkLog10k_1000NoNulls
BenchmarkLog10k_1000NoNulls-12                        37          30940075 ns/op
BenchmarkLog10k_1SomeSeriesHalfNulls
BenchmarkLog10k_1SomeSeriesHalfNulls-12            43148             27560 ns/op
BenchmarkLog10k_10SomeSeriesHalfNulls
BenchmarkLog10k_10SomeSeriesHalfNulls-12            4471            293390 ns/op
BenchmarkLog10k_100SomeSeriesHalfNulls
BenchmarkLog10k_100SomeSeriesHalfNulls-12            384           3079671 ns/op
BenchmarkLog10k_1000SomeSeriesHalfNulls
BenchmarkLog10k_1000SomeSeriesHalfNulls-12            40          29678756 ns/op
BenchmarkLog10k_1AllSeriesHalfNulls
BenchmarkLog10k_1AllSeriesHalfNulls-12             42213             28313 ns/op
BenchmarkLog10k_10AllSeriesHalfNulls
BenchmarkLog10k_10AllSeriesHalfNulls-12             4590            256792 ns/op
BenchmarkLog10k_100AllSeriesHalfNulls
BenchmarkLog10k_100AllSeriesHalfNulls-12             392           3157942 ns/op
BenchmarkLog10k_1000AllSeriesHalfNulls
BenchmarkLog10k_1000AllSeriesHalfNulls-12             37          30260522 ns/op
PASS
ok      github.com/grafana/metrictank/expr      18.915s

[zlin179]

Closed, will submit another one from bloomberg/metricTank