Better iteration

[mdorier]

Currently, iterating over a collection is done with the following backend function:

using DocIterCallback = std::function<Status(yk_id_t, const UserMem&)>;
virtual Status docIter(
    const char* collection,
    int32_t mode, uint64_t max, yk_id_t from_id,
    const std::shared_ptr<DocFilter>& filter,
    const DocIterCallback& func) const;

This should be changed so that the DocIterCallback can be called with an std::vector if the backend is able to provide all the UserMem in one batch (or in fewer batches). The filter function should also expose a shouldStop method so that one can specify UINT64_MAX as the max number of items, and the filter decides when to stop based on the key/doc.