Best-Effort durability semantics

Currently the Python SDK allows one to specify a `-1` value for either `persist_to` or `replicate_to`. When this is specified, the SDK will attempt to use the maximum number of servers/replicas available for durability requirements.

In pseudo code, it might look like the following

``` python
def get_durability_requirements(self, persist_to, replicate_to):
max_persist = self.replica_count + 1
max_persist = min(max_persist, self.nodes_online)

max_replica = max_persist - 1

if persist_to < 0:
persist_to = max_persist
if replicate_to < 0:
replicate_to = max_replica

return persist_to, replicate_to
```

This basically allows users to worry less about the topology of their cluster, with the assumed risk that if there are no replicas online then they would be running without a replica copy.