…ogging is disabled

[Why]
When the exchange logger handler is added, the broker is not ready to
declare the exchange.

The previous implementation used a temporary process that entered a loop
to wait for the broker to be ready and declare the exchange. It was
possible that definitions were imported before the exchange was
declared, leading to errors if bindings referenced it.

There was another problem: the exported definitions were dependent on
the exchange logger configuration:
* The default vhost under which the logger exchange is declared is
  configurable.
* The logger exchange is only declared if the exchange logger handler is
  enabled in the configuration.

Because of this, the import of definitions could fail if the
configuration of the node where the definitions are imported does not
enable the exchange logger handler.

[How]
This patch changes two things:

1. The logger exchange is always declared, regardless if exchange-based
   logging is enabled or not. This also allows to get rid of the polling
   in the setup process.

2. The logger exchange is included in the exported definitions.

This way, the exported definitions don't depend on any configuration on
the nodes they are imported in: the exported definitions are
self-contained and know about this logger exchange.

Part 1 could be enough to fix the problem on RabbitMQ version that
include this patch. Part 2 allows to fix the problem when definitions
are exported from a version with this patch and are imported on a
version without.

V2: Ignore the failure to declare the exchange. It should probably not
    be the only logger configured because there is a chicken-and-egg
    loop with it. It should not block the broker from starting.