Clean up `SamplingStrategy` APIs

[qkniep]

There are some problems with the current API, specifically:

• Strategies that only make sense for quorums, like FaitAccompli1Sampler, need to implement sample() to sample a single node.
• Right now sample_multiple() always takes the number of samples k as parameter, although some strategies, like FaitAccompli1Sampler, need to be initialized for a specific k. Either the samplers should be instantiated for a specific quorum size, and then sample_quorum() should always yield a quorum of that size, or these samplers should handle a way of initializing more than one quorum size (or even lazily initializing quorums when sample_multiple() is called with a new value for k).