test_astobj2_thrash: Fix multithreaded issues

The test uses 4 threads to grow, count, lookup and shrink 15K objects
in a container.  If there's only 1 execution engine available, the test
will complete in <50ms.  If each threads gets its own execution engine,
the test may timeout after 60 seconds because the count thread does a
locked ao2_callback on the whole container in a tight loop with only
a sched_yield to give up time.  The lock contention makes the test
execution times wildly variable and mostly timeout.  2 execution
engines are OK, 3 results in about 33% failure rate and >=4 causes
a 80% failure rate.

To fix, the sched_yield was changed to a usleep(500).

Also, the number of buckets specified for the container was an even
number so that was changed to the next prime number greater than
(MAX_HASH_ENTRIES / 100).  That's 151 currently.

Change-Id: I50cd2344161ea61bfe4b96d2a29a6ccf88385c77