Use max compression level in ZSTD

[jarifibrahim]

The default level is 5. This PR sets the compression level to 15 which gives the best speed vs compression ratio trade-off.

Level	Compression Ratio	Time Taken (Data Dog) (in n/s)
1	2.932754881	21152
2	2.753564155	21752
3	2.729475101	24112
4	2.816666667	38338
5	2.897142857	46792
6	2.954115076	69259
7	2.954115076	94859
8	2.954115076	122494
9	2.954115076	76904
10	2.954115076	75655
11	2.870488323	178359
12	3.09382151	418902
13	4.384864865	244562
14	4.384864865	242424
15	4.384864865	239042
16	4.38961039	471493
17	4.38961039	470330
18	4.38961039	472117
19	4.38961039	471703
20	4.38961039	474596

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This change is 

[manishrjain]

Got a comment.

Reviewed 1 of 1 files at r1.
Reviewable status: all files reviewed, 1 unresolved discussion (waiting on @ashish-goswami and @jarifibrahim)

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y/zstd_cgo.go, line 35 at r1 (raw file):

// ZSTDCompress compresses a block using ZSTD algorithm.
func ZSTDCompress(dst, src []byte) ([]byte, error) {
	return zstd.CompressLevel(dst, src, 15)

Add a big comment here about the benchmark results that you did.

[manishrjain]

Let's allow the user to decide in our options. Get another PR.

Reviewable status: all files reviewed, 1 unresolved discussion (waiting on @ashish-goswami and @jarifibrahim)

[coveralls]

Coverage decreased (-0.01%) to 77.344% when pulling 3285418 on ibrahim/zstd-level into cbbaefa on master.

[manishrjain]

Reviewed 6 of 6 files at r2.
Reviewable status: all files reviewed, 1 unresolved discussion (waiting on @ashish-goswami and @jarifibrahim)