Place an upper bound on notification concurrency

In Cbench tests we have seen as many as 2000 threads being created. This
is certainly an overkill and thus let's place an upper bound on the
number of threads we create.

Change-Id: I6a7f429080504cbfa75679cc5879264a0622f140
Signed-off-by: Robert Varga <rovarga@cisco.com>