Enable alternative zsync download strategies

[batkinson]

When syncing sparsely matching content, it can actually be more expensive to use incremental sync than it would be to just download the whole file directly. This is particularly true when using HTTP compression, since multi-range requests aren't compressed.

Provide an effective means to handle content like this. This could be as simple as aborting the sync with a certain exception so the client can handle directly, or the sync could attempt to handle it by allowing users to supply alternative strategies or handling it as part of the library itself.