Refactor database logging backend

[hagertnl]

To prepare for other database logging, we need to abstract the InfluxDB logging code out of the harness core code.

Initial thoughts are that we can create a class called something like db_logger, which can enable multiple database backends simultaneously, and receives a dictionary of the test data (or metrics or nodescreen) that it logs to the respective databases.

This will enable us to go and add a Kafka database backend.

[hagertnl]

Note -- when we get to the metrics & node screen data, can we try to bundle messages together please? :) We did not in the original implementation, but when we run a 1000-node node screen, it currently has to log 1000 messages to InfluxDB, instead of 1 large message that has 1000 insertions. I don't know the API for this off the top of my head, but it should be possible.