dirty_memory_manager: flush_permit: release_sstable_write_permit: mark noexcept
dirty_memory_manager: flush_permit: release_sstable_write_permit: mark noexcept

Neither exchanging the std:;optional nor moving
the sstable_write_permit throw.

Signed-off-by: Benny Halevy <23c997c90eb8537635e9392d48e7af1bf1b2ca22@scylladb.com>

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