Check conflict key before updating expiration map.

[martinmr]

When setting keys, the check of the conflict key is happening
after the expiration map is updated but the order of these two
operations should be reversed.

Also refactor the logic in this method for easier readability.

Fixes #149

---

This change is 

[manishrjain]

Reviewed 1 of 1 files at r1.
Reviewable status: all files reviewed, 1 unresolved discussion (waiting on @jarifibrahim, @karlmcguire, and @martinmr)

---

store.go, line 164 at r1 (raw file):

	if ok {
		// The item existed already. We need to check the conflict key and reject the
		// update if they do not match. The nwe update the expiration map.

"The ??? update?" -- fix this sentence.

[martinmr]

Reviewable status: 0 of 1 files reviewed, 1 unresolved discussion (waiting on @jarifibrahim, @karlmcguire, and @manishrjain)

---

store.go, line 164 at r1 (raw file):

Previously, manishrjain (Manish R Jain) wrote…

"The ??? update?" -- fix this sentence.

Done.