test/librados/aio: wait for all completions properly

test/librados/aio: wait for all completions properly

Using global semaphores was error-prone and unnecessary. Unbalanced
usage resulted in a race like:

aio_write object foo -> dropped on full OSD
aio_read object foo -> -ENOENT

The intent of the test was to wait for the write to complete, so
do that explicitly for each aio test and remove the semaphores.

Fixes: http://tracker.ceph.com/issues/24587
Signed-off-by: Josh Durgin <jdurgin@redhat.com>