collection_mutation: don't linearize collection values

[michoecho]

Yet another patch preventing potentially large allocations.
Currently, collection_mutation{_view,}_description linearize each collection
value during deserialization. It's not unthinkable that a user adds a
large element to a list or a map, so let's avoid that.

This patch removes the dependency on linearizing_input_stream, which does not
provide a way to read fragmented subbuffers, and replaces it with a new
helper, which does. (Extending linearizing_input_stream is not viable without
rewriting it completely).

Only linearization of collection values is corrected in this patch.
Collection keys are still linearized. Storing them in managed_bytes is likely
to be more harmful than helpful, because large map keys are extremely unlikely,
and UUIDs, which are used as keys in lists, do not fit into manages_bytes's
small value optimization, so this would incure an extra allocation for every
list element.

Note: this patch leaves utils/linearizing_input_stream.hh unused.

Refs: #8120

[denesb]

Note: this patch leaves utils/linearizing_input_stream.hh unused.

Feel free to send a patch to remove it completely, I don't think we need it anymore, it was never meant to be more than a temporary bridge to the land of fragmented buffers.