MEDIUM: bwlim: Use a read-lock on the sticky session to apply a shared limit

MEDIUM: bwlim: Use a read-lock on the sticky session to apply a shared limit

There is no reason to acquire a write-lock on the sticky session when a
shared limit is applied because only the frequency is updated. The sticky
session itself is not modified. We must just take care it is not removed in
the mean time. So a read-lock may be used instead.