Enable limits on function replication

[vsreekanti]

The policy engine currently just takes in latencies and throughputs and makes guesstimates about what the replication of a function should be. Sometimes, the user will know better. If the user wants to provide hints (e.g., never less than 2, never more than 8), we should enable them to do that (which will require changing the Cloudburst API) and abide by it.