unit tests for ObjectContext read/write locks

unit tests for the ObjectContext methods ondisk_write_lock,
ondisk_write_unlock, ondisk_read_lock and ondisk_read_unlock.

A class derived from ::testing::Test is created with two sub-classes (
Thread_read_lock & Thread_write_lock ) to provide a separate thread
that can block with cond.Wait(). usleep(3) is used in the main thread
to wait for the expected side effect with increasing delays ( up to
MAX_DELAY ).

http://tracker.ceph.com/issues/5487 refs #5487

Signed-off-by: Loic Dachary <loic@dachary.org>
Merge pull request #407 from dachary/wip-5487

unit tests for ObjectContext read/write locks

Reviewed-by: Samuel Just <sam.just@inktank.com>