BUG/MINOR: server/idle: at least use atomic stores when updating max_used_conns

BUG/MINOR: server/idle: at least use atomic stores when updating max_used_conns

In 2.2, some idle conns usage metrics were added by commit cf612a045
("MINOR: servers: Add a counter for the number of currently used
connections."), which mentioned that the operation doesn't need to be
atomic since we're not seeking exact values. This is true but at least
we should use atomic stores to make sure not to cause invalid values
to appear on archs that wouldn't guarantee atomicity when writing an
int, such as writing two 16-bit words. This is pretty unlikely on our
targets but better keep the code safe against this.

This may be backported as far as 2.2.