test_roscpp: added unit test for calling zombie services, see #434

Zombie services are left behind by crashed nodes. Currently roscpp can
lock up when you call such a zombie service.
test_roscpp: service_call_zombie: reduce service call count

This makes the test faster if it passes.
roscpp: Connection: emit drop_signal_ after unlocking drop_mutex_

Callbacks connected to the signal might want to lock other things, which
will violate lock ordering and probably cause a deadlock.

Moving the call out of the protected section is not dangerous, since
the connection is only dropped once.
roscpp: ServiceManager: initialize connection before connecting, fixes #434

This makes sure that the drop callback is registered before the connect
occurs. Previously it was possible to loose the drop event and cause a
freeze of the current thread.
Merge pull request #435 from xqms/zombie_lockup

test_roscpp: added unit test for calling zombie services, see #434