Add multi-thread support

[edmocosta]

• Added multi-thread support to logstash-input-snmp and logstash-input-snmptrap plugins:

  • logstash-input-snmptrap: Uses the SNMP4J MultiThreadedMessageDispatcher, processing multiple trap messages in parallel. By default, SNMP4J uses a single listener thread, which delegates all received data to this multi-threaded message dispatcher.

  • logstash-input-snmp: Uses a thread pool to submit the hosts requests, avoiding blocking the main thread, executing and aggregating multiple requests concurrently.

  • Increasing the MultiThreadedMessageDispatcher threads has almost no impact on the overall performance, as request are still serialised. This plugins uses the default value of 1.

  • A few SNMP results-related logs were moved to the request aggregator.

• Bumped SNMP4J version to 3.8.0 (latest). It has fixed a few concurrency issues (especially with TCP) and fixed/delivered several improvements. The current version (2.8) was the one originally used by the SNMP input plugin, and was released in 2020.

[edmocosta]

Hi @jsvd! thank you so much for reviewing! I've addressed your suggestions, could you please have a look?