Add new method to iterate a bufferlist without having to allocate an iterator.
Add convenience method for getting an item from the list based on the group and
index.
Remove redundant _do_data callback and method.
Update unit-tests and add some more for the new methods.
Buffer lists are a means to manage disjoint buffers as one buffer. It's also
possible to put many of those buffers into a list.
The idea is that when support is added to various elements, we will be able to
more efficiently slice and dice buffers, reduce the amount of memcpy and also
reduce data passing overhead.
The implementation is kept simple on purpose, reusing all of the memory
management features we have for miniobjects and buffers.
Access to the bufferlist object is done with an iterator, which allows for
efficient iteration and modification of the list.
See #572285
