use stream writer for full snapshot transfer

[gitlw]

Tested through the snapshot_test.go

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This change is 

[manishrjain]

Couple of comments. Definitely remove the Flatten call. Address those before merging.

Reviewed 21 of 21 files at r1.
Reviewable status: all files reviewed, 2 unresolved discussions (waiting on @gitlw and @martinmr)

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worker/snapshot.go, line 61 at r1 (raw file):

	var writer badgerWriter
	if snap.SinceTs == 0 {
		//Before we write anything, we should drop all the data stored in ps.

Add space between // and B.

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worker/snapshot.go, line 103 at r1 (raw file):

	glog.V(1).Infof("Flushed all writes to Badger. Flattening it now.")
	if err := pstore.Flatten(1); err != nil {

Can remove this flatten stuff. Unnecessary.

[manishrjain]

Reviewed 1 of 1 files at r2.
Reviewable status: all files reviewed, 1 unresolved discussion (waiting on @gitlw and @martinmr)

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worker/snapshot.go, line 62 at r2 (raw file):

	if snap.SinceTs == 0 {
		// Before we write anything, we should drop all the data stored in ps.
		if err := pstore.DropAll(); err != nil {

Remove.