* replacer: evaluate format once and improve perf by ~3x

This improves the performance of logging by almost 3x and reduces memory
usage by ~8x.

Benchmark results:

benchmark                                old ns/op     new ns/op     delta
BenchmarkReplacer-12                     644           324           -49.69%
BenchmarkReplacer_CommonLogFormat-12     4228          1471          -65.21%

benchmark                                old allocs     new allocs     delta
BenchmarkReplacer-12                     8              2              -75.00%
BenchmarkReplacer_CommonLogFormat-12     51             17             -66.67%

benchmark                                old bytes     new bytes     delta
BenchmarkReplacer-12                     240           48            -80.00%
BenchmarkReplacer_CommonLogFormat-12     3723          446           -88.02%

* replacer: code review comments

* bufPool: document why we use a pointer to a slice
* parseFormat: fix confusing comment
* TestParseFormat_Nodes: rename to TestParseFormatNodes

* replacer: use a value for bufPool instead of a pointer

* replacer: remove comment

* replacer: replace labels with knownLabels

The previous slice of label names is no longer needed.