timer: fix synchronization in stress test

timer: fix synchronization in stress test

Fix app/test timer stress test 2: Sometimes this test fails and
seg-faults because the slave lcores get out of phase with the master.
The master uses a single int, 'ready', to synchronize multiple slave
lcores through multiple phases of the test.

To resolve, we construct simple synchronization primitives that use one
atomic-int state variable per slave. The master tells the slaves when to
start, and then waits for all of them to finish. Each slave waits for
the master to tell it to start, and then tells the master when it has
finished.

Signed-off-by: Robert Sanford <rsanford@akamai.com>