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).
Small optimization. In some places we call `load` on a position
that is in the currently-held page. In those cases we are needlessly
calling `cached_file::get_shared_page` for the same page again,
adding some work and some noise in CQL tracing. This patch adds
an `if` against that.
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.
