Fix merge iterator duplicates issue

[jarifibrahim]

Fixes: #1153

The tree-based merge iterator would not skip duplicate keys. This PR fixes it.
If merge iterator consists of 3 iterator

it1 = [1, 1, 1]
it2 = [1, 1, 1, 1, 2]
it3 = [1, 1, 1, 1, 2]

The result should be

[1, 2]

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This change is 

[coveralls]

Coverage decreased (-0.1%) to 69.921% when pulling 9a52493 on ibrahim/mergeiterator-duplicate into 7c5e6d7 on master.

[pawanrawal]

please merge after tests pass.

Reviewed 2 of 2 files at r1.
Reviewable status: all files reviewed, 1 unresolved discussion (waiting on @ashish-goswami, @jarifibrahim, and @manishrjain)

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table/merge_iterator.go, line 33 at r1 (raw file):

	small *node

	curKey  []byte

Please add a comment saying that this is used for remove duplicate keys across different iterators.

[manishrjain]

Reviewed 2 of 2 files at r1.
Reviewable status: all files reviewed, 2 unresolved discussions (waiting on @ashish-goswami and @jarifibrahim)

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table/merge_iterator.go, line 164 at r1 (raw file):

func (mi *MergeIterator) setCurrent() {
	if cap(mi.curKey) < len(mi.small.key) {
		mi.curKey = make([]byte, 2*len(mi.small.key))

mi.curKey = append(mi.curKey[:0], mi.small.key...)

[jarifibrahim]

Windows build is not failing because of this PR. I'll fix the windows build in a separate PR.