r15555: Make "change notify timeout" a per-share parameter - used

r15555: Make "change notify timeout" a per-share parameter - used
when there's no kernel or FAM change notify. If set to zero
this will turn off change notify for the share except when
we ourselves change something (renames / deletes etc. ).
Designed to help on large directory shares where a new
changenotify is issued between each delete. This will
be fixed correctly when we move to internal change notify
(eg. back-port Samba4 changenotify).
Jeremy.
(This used to be commit 5a17bffbcd5082fde79c241468a0ff2b5903d540)