client: preserve ordering of readdir result in cache

client: preserve ordering of readdir result in cache

Preserve ordering of readdir result in a list, so that the result of cached
readdir is consistant with uncached readdir.

As a side effect, this commit also removes the code that removes stale dentries.
This is OK because stale dentries does not have valid lease, they will be
filter out by the shared gen check in Client::_readdir_cache_cb()

Signed-off-by: Yan, Zheng <zyan@redhat.com>