erofs-utils: introduce buffer cache

This patch adds a buffer cache mainly to manage
incomplete metadata blocks. mkfs logic is also
simplified with the help of buffer cache.

Signed-off-by: Miao Xie <miaoxie@huawei.com>
[ Gao Xiang: with heavy changes. ]
Signed-off-by: Gao Xiang <gaoxiang25@huawei.com>