[roll] Roll fuchsia [fxfs] Avoid boxing future in LayerIterator

When advancing layer iterators through trait objects, such as when the
merger advances sub-iterators, we frequently allocate boxed futures even
when the underlying iterator can advance synchronously (e.g. for
in-memory skip lists or cached persistent layer chunks).

Allow dynamic layer iterators to complete advance operations
synchronously without allocating heap futures on the hot path.

TAG=agy
CONV=54607f32-26db-4cb9-b07f-77438a8a2b02

Test: fx test fxfs-tests
Original-Original-Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1587188
Original-Original-Revision: 1e8fbd66f7d2c753608e1685657a5b567b72d4ae
GitOrigin-RevId: a31809475da5344970519c2c156420f0fc7b1987
Change-Id: I2ca9c38e3cd690cd950e162d679336e842bc7937
Reviewed-on: https://fuchsia-review.googlesource.com/c/integration/+/1794277
Cr-Commit-Position: refs/heads/main@{#200780}