Improve processlist scraping with the transaction duration

[Uyoooo]

Hello @SuperQ,

This PR add a new metric mysql_info_schema_max_tx_duration with the max transaction duration for each thread state.

Inspired by https://github.com/prometheus-community/postgres_exporter/blob/master/cmd/postgres_exporter/postgres_exporter.go#L305

mysql_info_schema_max_tx_duration{state="after create"} 0
mysql_info_schema_max_tx_duration{state="altering table"} 0
mysql_info_schema_max_tx_duration{state="analyzing"} 0
mysql_info_schema_max_tx_duration{state="checking permissions"} 0
mysql_info_schema_max_tx_duration{state="checking table"} 0
mysql_info_schema_max_tx_duration{state="cleaning up"} 0
mysql_info_schema_max_tx_duration{state="closing tables"} 0
...

This PR can resolve #556

[chicocvenancio]

We have a need for this metric as well, would rebasing this into the current codebase be enough to get this merged?