cmd/faucet: protect f.reqs with Rlock to prevent data race

[BurtonQin]

In cmd/faucet/faucet.go,
f.reqs is protected by f.lock.Lock() or f.lock.Rlock() at five places.
The only place where it is not protected is on L367 in func apiHandler().
This may cause data race when apiHandler() and other func like refresh() where f.reqs is written to are called in parallel.
The fix is to use f.lock.Rlock() to protect f.reqs.

[karalabe]

Thanks for the PR, however this will lock the faucet for the entire duration of the network transmission, which is not a good idea. If f.reqs needs to be protected, we need to make a locked copy and then transmit with the lock released.

[karalabe]

This change does not do a deep copy of f.reqs, so it's not ultra safe, but we only ever append and slice it (never update an entry), so it should be fine.