Memory store putSync performance improvement

[msssk]

Fixes #183

The existing logic splices out a modified element and splices it back in
any time an update is made with putSync. With large arrays the performance
of Array#splice is bad. Additionally, the store's index is rebuilt to account
for moved items.

This PR adds a check to see if the item being put has moved. If not, the array
is modified in-place with no splicing, and the store's index is not rebuilt.

Some boundary tests for putSync are also added to the test suite.

Thanks to @nijuyr for the troubleshooting and approach.

[nijuyr]

I'm happy that the fix is in place.
However it is odd that not all suggested changes were included.
In The PR I made, not only the fix for the issue was included but it also cleared up things with some improvements in the overall code maintainability & readability.

1. No arbitrary options check in functional lines of code by evaluating options first.
2. Clear comments for where important logic is placed

I'd sure not be happy seeing code such as
L101

					if (eventType === 'update') {
						--destination;
					}

Again, thanks for having the fix in but I believe the next update can include some code improvement as well.