DUP: stop custom handling of AMQP connections

[Annopaolo]

AMQPDataConsumer leaked channels when the process did not terminate correctly. Replace the ad-hoc handling of connections and channels with the ExRabbitPool pooling library. Contexually, add an env var to customize the number of connections to the broker:

• DATA_UPDATER_PLANT_AMQP_CONSUMER_CONNECTION_NUMBER (default: 10)

As DUP is still using one channel per queue, the number of channels per connection is explicitly computed as ⎡total_queue_number/DATA_UPDATER_PLANT_AMQP_CONSUMER_CONNECTION_NUMBER⎤.

[codecov[bot]]

Codecov Report

Attention: 35 lines in your changes are missing coverage. Please review.

Comparison is base (d79263b) 67.49% compared to head (b57bf46) 67.51%. Report is 3 commits behind head on release-1.1.

Files	Patch %	Lines
...b/astarte_data_updater_plant/amqp_data_consumer.ex	42.50%	23 Missing :warning:
...astarte_data_updater_plant/amqp_events_producer.ex	52.17%	11 Missing :warning:
...ter_plant/lib/astarte_data_updater_plant/config.ex	85.71%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## release-1.1 #895 +/- ## =============================================== + Coverage 67.49% 67.51% +0.02% =============================================== Files 264 263 -1 Lines 6439 6434 -5 =============================================== - Hits 4346 4344 -2 + Misses 2093 2090 -3 ```

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