Support per-member ReplicationFactor

[park-bit]

Closes #26

so rn every member just uses the same ReplicationFactor from Config,
no way to give a bigger/heavier node more of the ring than a small one.

added a small optional interface for this:

type ReplicationFactorMember interface {
Member
ReplicationFactor() int
}

if a member implements it, Add/Remove will use its own factor instead
of the global one. if it doesn't, nothing changes, falls back to
config.ReplicationFactor like before. so shouldn't break anything.

also added a test for it, checks the vnode count matches what we'd
expect and that Remove actually cleans up all of them after.

ran everything with -race too, all passing on my end