librgw: implement small-directory name cache

librgw: implement small-directory name cache

Augment the existing marker cache with cache of name->type
mappings for up to 128 dirents (appx. 4x the common upper-bound
of 32).

The motivation here is to use the name and type information from
readdir, rather than discarding it.  Boosts perf. at least 60%
on unit test corpus.

Signed-off-by: Matt Benjamin <mbenjamin@redhat.com>