455184: Memory Leak: MultiClientSessionCookieStore hangs on to
ClientSessions
Convert to WeakHashMap. Add ReadWriteLock for better performance.

Change-Id: I8ad441c8ca117142105e121c08dc052091292e42
Task-Url: https://bugs.eclipse.org/bugs/show_bug.cgi?id=455184
Signed-off-by: Patrick Bänziger <patrick.baenziger@bsiag.com>
Reviewed-on: https://git.eclipse.org/r/38339
Tested-by: Hudson CI
Reviewed-by: Judith Gull <jgu@bsiag.com>