added locking to mutation methods Store, DeleteOne and DeleteAll

[nutbunnies]

Fixes issues with issue mailhog/MailHog#121. Running tests without mutex locks and -race will produce the same fatal error: concurrent map writes error. I'm very new to Go so this might not be the best solution.

[ian-kent]

👍 thanks @nutbunnies, looks good! if you can fix the mutex unlock thing I'll get it merged 😄

[ian-kent]

this should be a defer memory.mu.Unlock() after line 27 instead, e.g.

memory.mu.Lock()
defer memory.mu.Unlock()

although unlikely, if either line 28 or 29 panics, the mutex would still be locked and would cause all other goroutines to block (see https://gist.github.com/ian-kent/06efa5b60d3b9f54ee07648e770a1958 for an example)

the same applies to the other mutex unlocks in the rest of memory.go

[nutbunnies]

GTK on the defer stuff; I updated the 3 usages to use defer.

[ian-kent]

👍 awesome, thanks @nutbunnies