Improve putSync performance of Memory store.

[nijuyr]

Closes #183

This PR improves the performance of putSync by

1. skipping splice operation on array when the index of the updated object is not changed. (most of simple object update via putSync does not change index)

• Array.prototype.splice is an expensive operation especially when the size of the array is huge.

2. skipping re-indexing when the index of the updated object is not changed.

Performance -
Updating 10000 objects with putSync takes ~15ms

[dylans]

Thanks @nijuyr , we will review this in early January and hopefully land this and then get a new dstore release live that includes this update.

[msssk]

This enhancement was made in #227. Thank you for your work on this @nijuyr!