vinyl: do not allow to cancel a fiber reading a page

To handle fiber cancellation during page read we need to pin all objects
referenced by vy_page_read_task. Currently, there's the only such
object, vy_run. It has reference counting so pinning it is trivial.
However, to move page lookup to a reader thread, we need to also
reference key def, tuple format, and key. Format and key have reference
counting, but key def doesn't - we typically copy it. Copying it in this
case is too heavy.

Actually, cancelling a fiber manually or on timeout while it's reading
disk doesn't make much sense with PCIE attached flash drives. It used to
be reasonable with rotating disks, since a rotating disk controller
could retry reading a block indefinitely on read failure. It is still
relevant to Network Attached Storage. On the other hand, NAS has never
been tested, and what isn't tested, can and should be removed. For
complex SQL queries we'll be forced to rethink timeout handling anyway.

That being said, let's simply drop this functionality.