lock Increment/Decrement to avoid lostupdate

[hidetatz]

In current implementation, if multiple goroutines calls Increment/Decrement simultaneously, the lost update will occur. Let's consider below scenario:

• The record "counter:0" exists in the NumberCache store
• Goroutine A and B want to increment that by 1
  • After all, the record should be "counter:2"
• GetItem called by Goroutine A returns "counter:0"
• GetItem called by Goroutine B returns "counter:0"
• Goroutine A increments it by 1, so the record is now "counter:1"
• Goroutine B increments it by 1, but the record is still "counter:1" because Goroutine B incremented stale record "counter:0"

I have written unit tests to make sure above behavior occurs. You will see tests fail if you pull this branch locally and comment out the changes in cache.go, then run gotip test .

[Code-Hex]

@hidetatz Thanks!