Introduce amqp_connection:register_blocked_handler

For handling of connection.blocked and connection.unblocked
methods. Mimics the channel.flow handling API.

Incomplete, does not yet pass the tests. Pushed
for review.
Add connection.blocked to client capabilities
Print where test output is redirected

Also, provide an alternative task that runs tests in foreground,
useful when debugging a test.
Correct connection.blocked test to use set/clear alarm

It was originally based on an old channel.flow test that
did not run and wouldn't work on RabbitMQ versions after 2.0.

This implementation sets and clears alarms the same way
server tests in the Java client do.

Makes make test_network pass.
Limit this connection.blocked test to network connections

Still exploring if it's a good idea to have two separate tests
or not.
Merge default into bug25191
Untabify
Remove a confusing log line left from an old QoS test
Merge bug25191