db/hints/resource_manager: Ensure managers are correctly registered
db/hints/resource_manager: Ensure managers are correctly registered

Registering a manager for a new device used
std::unordered_map::emplace(), which may not insert the specified
value if one with the same key has already been added. This could
happen if both managers were using the same device and the fiber
deferred in-between adding them.

Found during code reading. Could cause hints to not be disabled for an
overloaded manager.

Fixes #3822

Signed-off-by: Duarte Nunes <a60e8b7496f7715d71354fe4c06dd24017025bf1@scylladb.com>

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