feat(Dgraph): Add ability to change size of caches through the admin interface.

[martinmr]

The endpoints have been changed from lruMb to cacheMb to reflect that this
is changing the value of the cacheMb flag.

Fixes DGRAPH-2515

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This change is 

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[martinmr]

CI is failing because the cache sizes need to be properly initialized. Once branch martinmr/wal is merged into master, CI should start working again.

[manishrjain]

Ensure build is still green after this.

Reviewed 16 of 16 files at r1.
Reviewable status: all files reviewed, 1 unresolved discussion (waiting on @danielmai, @martinmr, @MichaelJCompton, @pawanrawal, and @vvbalaji-dgraph)

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worker/worker.go, line 131 at r1 (raw file):

// UpdateCacheMb updates the value of cache_mb and updates the corresponding cache sizes.
func UpdateCacheMb(memoryMB int64) error {
	glog.Infof("memoryMB %d", memoryMB)

Updating cacheMB to ...

[codelingo]

LGTM. 8 rules errored during the review.

[codelingo]

LGTM. 8 rules errored during the review.