The flag protects etcd memory from being swapped out to disk.
This can happen in memory constrained systems where mmaped bbolt
area is natural condidate for swapping out.
This flag should provide better tail latency on the cost of higher RSS
ram usage. If the experiment is successful, the logic should get moved
into bbolt layer, where we can protect specific bbolt instances
(e.g. avoid protecting both during defragmentation).
Because of the well-known range loop closure issue, the value of u may
have changed by the time the anonymous function mentioned in the defer
is run. To address this, the simplest fix is to pass the url used in the
loop as an argument to the function run in defer.
