compression: add an adaptive compressor

[RaduBerinde]

Informs #4925

compression: return Setting from Compress

Flip-flopping on my earlier change :)

ewma: add a per-bye EWMA estimator

This will be used to estimate compression ratio of blocks based on the
compression ratios of recently sampled blocks.

compression: add an adaptive compressor

Add AdaptiveCompressor, which estimates the size reduction of using a
slower algorithm vs a faster one and chooses automatically (on a
per-block basis).

We will separately update the compression analyzer to run experiments
with the adaptive compressors.

compressionanalyzer: support adaptive compressors

Switch experiments from using compression.Setting to
block.CompressionProfile and add two adaptive profiles.

[cockroach-teamcity]

This change is 

[annrpom]

cool!

Reviewed 10 of 10 files at r1, 2 of 2 files at r2, 4 of 4 files at r3, 8 of 8 files at r4, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @jbowens)

[RaduBerinde]

TFTR! Some initial tests on TPCC data show it is a pretty decent trade-off between MinLZ and Zstd1, especially in decompression speed. This is for ~32KB data blocks, broken down by the "compressibility" of the block (as determined by MinLZ). The cutoff is set to 30%:

 Test CR           MinLZ1            Zstd1             Auto1

 <1.1      CR      1.02 ± 3%         1.41 ± 3%         1.28 ± 15%
           Comp    3633MBps ± 86%    364MBps ± 19%     533MBps ± 59%
           Decomp  19072MBps ± 84%   1055MBps ± 27%    2002MBps ± 71%

 1.1-1.5   CR      1.31 ± 8%         1.56 ± 9%         1.31 ± 8%
           Comp    1194MBps ± 24%    370MBps ± 15%     903MBps ± 77%
           Decomp  5765MBps ± 31%    980MBps ± 16%     5710MBps ± 32%

 1.5-2.5   CR      1.75 ± 12%        2.17 ± 15%        1.75 ± 13%
           Comp    1254MBps ± 42%    427MBps ± 27%     977MBps ± 80%
           Decomp  3664MBps ± 42%    1010MBps ± 28%    3672MBps ± 44%

 >2.5      CR      5.71 ± 82%        9.42 ± 66%        8.30 ± 70%
           Comp    1719MBps ± 46%    578MBps ± 26%     799MBps ± 57%
           Decomp  3509MBps ± 40%    1287MBps ± 34%    1979MBps ± 59%