allow instantiation of Cluster and RendezvousHash objects with custom murmur3 seed

@dstufft rightly pointed out that if this library were used in such a way that untrusted input were used for keys, murmur3 is susceptible to DoS attacks with specifically crafted keys.

this exposes an interface for setting the murmur3 seed for `Cluster` and `RendezvousHash` objects.