Both scatterGather() and establishCursors() should allow the caller to delegate deciding which shards to target and building versioned or unversioned requests to those shards.

The caller should just specify a targeting policy (BroadcastToAllShards, UseRoutingTable, etc) and any additional options if relevant (e.g., target for a specific query when using the routing table).

The current interface is muddled, and doesn't capture the range of targeting behavior used on mongos.

Rather than allow lots of if/else blocks, boost::optional arguments, and re-implementations of targeting logic to sprawl across the codebase, we should succinctly express different types of shard targeting policies and the options that go with each, through a clearly differentiated set of functions or a set of classes.

Some factors that determine a targeting policy and whether requests should be versioned:

• whether the command has an associated collection
• whether the associated collection is logically part of the routing table (as opposed to, say, $cmd or $aggregate)
• whether the command is retryable (e.g., writes are currently not)