Disable cache by default

[jarifibrahim]

Disabling cache by default leads to better performance when compression
and encryption is disabled.

Result of read bench tool with key size 32 bytes and value size 128 bytes
(Note: data is not compressed or encrypted)

1. With 1 GB cache:
       	Average read speed              : 41.3833 MB/s
      	Total bytes read in 1 minute    : 2.4 GB

2. Without cache:
	Average read speed		: 58.86 MB/s
	Total bytes read in 1 minute	: 3.3 GB

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This change is 

[manishrjain]

Reviewed 4 of 4 files at r1.
Reviewable status: all files reviewed, 1 unresolved discussion (waiting on @ashish-goswami and @jarifibrahim)

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table/builder_test.go, line 60 at r1 (raw file):

		for _, opt := range opts {
			builder := NewTableBuilder(opt)
			filename := fmt.Sprintf("%s%c%d.sst", os.TempDir(), os.PathSeparator, rand.Int63())

Int63 can return a negative number?

[jarifibrahim]

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

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table/builder_test.go, line 60 at r1 (raw file):

Previously, manishrjain (Manish R Jain) wrote…

Int63 can return a negative number?

rand.Int63() does not return a negative number

Int63 returns a non-negative pseudo-random 63-bit integer as an int64.

https://golang.org/pkg/math/rand/#Rand.Int63