The fs.* files are already part of the libbitcoin_util library. With the
introduction of the fs_helpers.* it makes sense to move fs.* into the
util/ directory as well.
Also introduce node::{ShouldPersistMempool,MempoolPath} helper functions
in node/mempool_persist_args.{h,cpp} which are used by non-kernel
DumpMempool callers to determine whether or not to automatically dump
the mempool and where to dump it to.
