Only grab a read-lock in Now().

[tsuna]

I can see two possible stances here:

1. You consider the fakeClock non-performance sensitive test code, in which case we might just turn the RWMutex into a plain Mutex.
2. You want to keep a RWMutex so that multiple callers can call Now() with less friction, in which case we should use RLock() / RUnlock().

I initially went with approach 1, somehow changed my mind for 2, but could be convinced either way.

[jonboulle]

Good spot. This is definitely my preferred approach. Thanks for the patch!