ovs-vswitchd: Fix high cpu utilization when acquire idl lock fails.

When ovs-vswitchd fails to acquire the ovsdb idl lock (either due to
contention or due to invalid database path), ovs-vswitchd will spin
on the global connectivity sequence number and consume 100% cpu.
This is in that the local copy is different to the global sequence
number and never updated when ovsdb idl is not locked.

To fix this issue, this commit makes ovs-vswitchd not checking the
global connectivity sequence number in that situation.

Reported-by: Ben Pfaff <blp@nicira.com>
Signed-off-by: Alex Wang <alexw@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>