EE license fixes.

[parasssh]

Fixes DGRAPH-1341

The enterprise_license option on zero is not applied properly. There were couple issues: 1) It would be over-written by trial license. 2) It generally failed because when the license was applied, Raft was not initialized and the zero was not the leader.

This patch make several fixes and improvements

1. Apply the option enterprise_license only after the node's Raft is initialized and it is the leader.
2. Don't apply the trial license if a license already exists.
3. Disallow the enterprise_license option for OSS build and bail out.
4. Apply the option even if there is a license from a previous life of the Zero (i.e. Zero is restarted with same the zw/

Tests:

1. New Cluster, dgraph zero
   Trial license is applied.
2. New Cluster, dgraph zero --enterprise_license=<path to EE license file>
   EE License is applied.
3. Existing Cluster, Previously Trial License applied., Now dgraph zero --enterprise_license=<path to EE license file>
   EE License is applied.
4. Existing Cluster, Previously EE License. Now dgraph zero
   Old EE License is used. Trial license is not applied.
5. OSS with enterprise_license option
   ERROR
6. OSS without enterprise_license option on New, Existing Cluster
   No Licenses applied.

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This change is 

[martinmr]

Reviewed 5 of 5 files at r1.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @danielmai and @manishrjain)

[manishrjain]

Just one comment.

Reviewable status: all files reviewed, 1 unresolved discussion (waiting on @danielmai and @parasssh)

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dgraph/cmd/zero/raft.go, line 746 at r1 (raw file):

			if !licenseApplied && n.AmLeader() {
				licenseApplied = true
				// Apply enterprise license if one was given.

You can mention that this can override the previous license that was present. That's an intended behavior because ...

[parasssh]

Dismissed @manishrjain from a discussion.
Reviewable status: 4 of 5 files reviewed, all discussions resolved (waiting on @danielmai and @martinmr)

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dgraph/cmd/zero/raft.go, line 746 at r1 (raw file):

Previously, manishrjain (Manish R Jain) wrote…

You can mention that this can override the previous license that was present. That's an intended behavior because ...

Done.