Support bulk-loader use-case to import unencrypted export and encrypt the result

[parasssh]

Fixes https://dgraph.atlassian.net/browse/DGRAPH-1254

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This change is 

[manishrjain]

Got comments.

Reviewed 3 of 3 files at r1.
Reviewable status: all files reviewed, 2 unresolved discussions (waiting on @parasssh)

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dgraph/cmd/bulk/loader.go, line 120 at r1 (raw file):

		writeTs:       getWriteTimestamp(zero),
	}
	st.schema = newSchemaStore(readSchema(opt.SchemaFile, opt.BadgerKeyFile, opt.Encrypted), opt, st)

just pass in the opt object, instead of passing in individual arguments.

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dgraph/cmd/bulk/run.go, line 109 at r1 (raw file):

			" The key size indicates the chosen AES encryption (AES-128/192/256 respectively). "+
			" This key is used to encrypt the output data directories and to decrypt the input "+
			" schema and data files (if enncrytped). Enterprise feature.")

if encrypted

[parasssh]

Reviewable status: 1 of 3 files reviewed, 2 unresolved discussions (waiting on @manishrjain)

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dgraph/cmd/bulk/loader.go, line 120 at r1 (raw file):

Previously, manishrjain (Manish R Jain) wrote…

just pass in the opt object, instead of passing in individual arguments.

Done.

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dgraph/cmd/bulk/run.go, line 109 at r1 (raw file):

Previously, manishrjain (Manish R Jain) wrote…

if encrypted

Done.

[parasssh]

Dismissed @manishrjain from 2 discussions.
Reviewable status: 1 of 3 files reviewed, all discussions resolved (waiting on @manishrjain)