Implement multiple locks at the same time.

[martinheidegger]

Right now, require('flexlock/createLocker') only allows a single key to lock towards lock(key, ...) but some code requires multiple resources to work, a new API that works like lock([keyA, keyB], ...) would be good. The obvious implementation would be to just use Promise.all to await three locks, but a more-complex implementation that allows other sublocks to receive early might be not a bad idea. In an example:

1. (A) lock(['users', 'user:11'], ...) → ... processing
2. (B) lock(['users', 'user:12'], ...) → ... waiting for (A) to finish
3. (C) lock('user:12') → processing
4. (A) finished, (B) → waiting for (C) to finish
5. (C) finished, (B) → processing
6. (B) finished

In that example (C) can start processing without waiting for (B) to finish because it only uses part of the resources of (B). This would require a more complicated implementation though.