Move checkpoint key to WAL

[manishrjain]

The checkpoint key was being stored in p directory. Can't think of a reason why I did that. Keeping the checkpoint key in WAL makes natural sense and allows someone to drop the w directory, without causing issues later. One of the issues is that if w is dropped, Raft index would start from 1 again, but p directory would have stored the progress to a much higher index. This would cause all new proposals to not be applied.

With this PR, w directory can be dropped and the checkpoint would also get dropped along with it.
This PR also adds two optimizations:

• Calculates checkpoints starting from the last checkpoint, instead of the first entry since snapshot.
• Does not calculate snapshot if the checkpoint - first < SnapshotAfter.

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This change is 

[martinmr]

Reviewed 4 of 6 files at r1, 1 of 2 files at r2.
Reviewable status: all files reviewed, 3 unresolved discussions (waiting on @manishrjain)

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worker/draft.go, line 716 at r2 (raw file):

					if first, err := n.Store.FirstIndex(); err == nil {
						// Save some cycles by only calculating snapshot if the checkpoint has gone
						// quite a bit further than first index.

nit: than the first index

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worker/draft.go, line 1172 at r2 (raw file):

// At i7, min pending start ts = S3, therefore snapshotIdx = i5 - 1 = i4.
// At i7, max commit ts = C1, therefore readTs = C1.
func (n *node) calculateSnapshot(startIdx uint64, discardN int) (*pb.Snapshot, error) {

Maybe edit the doc comment to include something about why startIdx is now being passed as an argument.

[manishrjain]

Reviewable status: 4 of 5 files reviewed, 2 unresolved discussions (waiting on @golangcibot and @martinmr)

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worker/draft.go, line 733 at r1 (raw file):

Previously, golangcibot (Bot from GolangCI) wrote…

Error return value of x.Errorf is not checked (from errcheck)

Done.

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worker/draft.go, line 1172 at r2 (raw file):

Previously, martinmr (Martin Martinez Rivera) wrote…

Maybe edit the doc comment to include something about why startIdx is now being passed as an argument.

Done.