directories: Keep a unique set of directories to initialize
directories: Keep a unique set of directories to initialize

If any two directories of data/commitlog/hints/view_hints
are the same we still end up running verify_owner_and_mode
and disk_sanity(check_direct_io_support) in parallel
on the same directoriea and hit #5510.

This change uses std::set rather than std::vector to
collect a unique set of directories that need initialization.

Fixes #5510

Signed-off-by: Benny Halevy <23c997c90eb8537635e9392d48e7af1bf1b2ca22@scylladb.com>
Message-Id: <20191225160645.2051184-1-23c997c90eb8537635e9392d48e7af1bf1b2ca22@scylladb.com>

scylladb/scylla,scylladb/scylla,avikivity/scylla,avikivity/scylla,avikivity/scylla,scylladb/scylla,scylladb/scylla