feat(Flags): [Breaking] Add flag for snapshot duration frequency

[rohanprasad]

Add a new flag snapshot-after-duration, which will take duration as a value. Raft snapshots will now be taken based on time since the last snapshot or the number of new raft entries. Also, rename snapshot-after to snapshot-after-entries for distinction.

Fixes DGRAPH-3213

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This change is 

[NamanJain8]

Some minor comments. Otherwise, looks good.

[manishrjain]

Reviewed 3 of 4 files at r1, 1 of 1 files at r2.
Reviewable status: all files reviewed, 6 unresolved discussions (waiting on @NamanJain8, @rohanprasad, and @vvbalaji-dgraph)

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dgraph/cmd/alpha/run.go, line 161 at r2 (raw file):

			`Make this Alpha a "learner" node. In learner mode, this Alpha will not participate `+
				"in Raft elections. This can be used to achieve a read-only replica.").
		Flag("snapshot-after-entries",

Just keep both for now. But, make it an AND. Both conditions must be true.

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dgraph/cmd/alpha/run.go, line 166 at r2 (raw file):

				"snapshot-after-duration or snapshot-after-entries threshold is crossed.").
		Flag("snapshot-after-duration",
			"Frequency at which we should create a new raft snapshots. Set "+

No need to repeat that "snapshots are created ...". Explain the format of duration -- 30m, 2h, ... Or, if it's in the help, then it's good.

snapshot-after

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worker/draft.go, line 1000 at r2 (raw file):

func (n *node) checkpointAndClose(done chan struct{}) {
	slowTicker := time.NewTicker(time.Minute)
	lastSnapshotTime := time.Now()

just keep this in memory.

[NamanJain8]

LGTM.
Few nit-picks. Otherwise, looks fine.