Convert sleep_for into count_subscriptions logic

[mjcarroll]

Feature request

Feature description

Converting from TODO for visibility:

rcl/rcl/test/rcl/test_subscription.cpp

Lines 178 to 180 in 94b5a1d

	// TODO(wjwwood): add logic to wait for the connection to be established
	// probably using the count_subscriptions busy wait mechanism
	// until then we will sleep for a short period of time

There are a few places in the test_subscription code (and possibly other places) where we make use of a fixed delay to wait for publications/subscriptions to become available. This should be converted to make use of count_publications/count_subscriptions when possible.

[Barry-Xu-2018]

@mjcarroll

I have an interest in implementing it.
I start to implement it.

[mjcarroll]

Thanks @Barry-Xu-2018! Looking forward to your pull request, let me know if you run into any issues.

[Barry-Xu-2018]

@mjcarroll

I have made PR.
Please review #631.

[jacobperron]

Looks like this was resolved in #631.