Merge function should process items in order

[jarifibrahim]

With this commit, merge operator processes the items in the order of which they were inserted. Earlier it would be processed by the merge function in reverse order.

For instance, if A , B, C were inserted, and merge function was simple append operation, they would be passed to the merge function as

mergeFunc(C, B) => result CB
mergeFunc(CB, A) => result CBA

With this change, they're passed to the merge function as

mergeFunc(B, C) => result BC
mergeFunc(A, BC) => result ABC

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This change is 

[manishrjain]

Got a comment.

Reviewed 1 of 2 files at r1, 2 of 2 files at r2.
Reviewable status: 2 of 3 files reviewed, 1 unresolved discussion (waiting on @ashish-goswami, @jarifibrahim, and @manishrjain)

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README.md, line 288 at r2 (raw file):

```Go
func uint64ToBytes(i uint64) []byte {

Don't remove this. It is important to show how to deal with integer addition.

You can just additionally add the ABC example to show how merge operator maintains ordering.

[jarifibrahim]

Reviewable status: 1 of 3 files reviewed, 1 unresolved discussion (waiting on @ashish-goswami and @manishrjain)

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README.md, line 288 at r2 (raw file):

Previously, manishrjain (Manish R Jain) wrote…

Don't remove this. It is important to show how to deal with integer addition.

You can just additionally add the ABC example to show how merge operator maintains ordering.

Done. Added the counter increment example to the readme.

[manishrjain]

Reviewed 1 of 1 files at r3, 1 of 1 files at r4.
Reviewable status: complete! all files reviewed, all discussions resolved

[campoy]

I'm reviewing all the PRs merged since our latest release and checking whether they impact our users.
I'd say this one doesn't, please let me know if that's not the case.