Before this patch, `reader_permit` taken by `bti_index_reader`.
wasn't actually being passed down to disk reads. In this patch,
we fix this FIXME by propagating the permit down to the I/O
operations on the `cached_file`.
Also, it didn't take `trace_state_ptr` at all.
In this patch, we add a `trace_state_ptr` argument and propagate
it down to disk reads.
(We combine the two changes because the permit and the trace state
are passed together everywhere anyway).
In later commits, a trie cursor will be holding a `page_ptr`.
Sometimes we will want to copy a cursor, in particular
to do reset the upper bound of the index reader with
`_lower = _upper`. But currently `page_ptr` is non-copyable
-- it's a shared pointer, but with an explicit `share()` method
-- so a default operator= won't work for this.
Let's add a copy-assignable for `page_ptr` for this purpose.
This commit implements routines for traversal of BTI nodes in their
on-disk format.
The `node_reader` concept is currently unused (i.e. not asserted by any
template).
It will only be used in the next PR, which will implement trie cursor
routines parametrized `node_reader`.
But I'm including it in this PR to make it clear which functions
will be needed by the higher layer.
