Security fix: prevent the HmacSecret from being logged

[gitlw]

This change is 

[martinmr]

Reviewed 2 of 2 files at r1.
Reviewable status: all files reviewed, 1 unresolved discussion (waiting on @gitlw and @manishrjain)

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edgraph/config.go, line 71 at r1 (raw file):

func (opt Options) String() string {
	//return fmt.Sprintf()
	return fmt.Sprintf("{PostingDir:%s BadgerTables:%s BadgerVlog:%s WALDir:%s MutationsMode:%d "+

minor: add spaces after the colon to make it easier to read.