ofproto: Do not delete datapath flows on exit by default.

ofproto: Do not delete datapath flows on exit by default.

Commit e96a5c24e853 ("upcall: Remove datapath flows when setting
n-threads.") caused OVS to delete datapath flows when it exits through
any graceful means.  This is not necessarily desirable, especially when
OVS is being stopped as part of an upgrade.  This commit changes OVS so
that it only removes datapath flows when requested, via "ovs-appctl
exit --cleanup".

Acked-by: Numan Siddique <numans@ovn.org>
Tested-by: Numan Siddique <numans@ovn.org>
Signed-off-by: Ben Pfaff <blp@ovn.org>