This changes forks the mysql part of the Macaron session middleware.

In the forked mysql file:

- takes in a config setting for SetConnMaxLifetime (this solves wait_timeout
problem if it is set to a shorter interval than wait_timeout)
- removes the panic when an error is returned in the Exist function.
- retries the exist query once
- retries the GC query once

For MySQL, setting this to be shorter than the wait_timeout MySQL setting
solves the issue with connection errors after the session has timed out for
the connection to the database via xorm.

Got added by mistake a year ago.

[ci skip]

Add color to prefix and postfix in singlestat panel

[ci skip]

* Seconds to HH:MM:SS format

[ci skip]

Prometheus heatmap support

Added concentration units and "Normal cubic metre"

docs: fix an outdated link to Prometheus's doc

ppm, ppb, ng/m3, ng/Nm3, μg/m3, μg/Nm3, mg/m3, mg/Nm3, g/m3, g/Nm3, Nm3
ppm was moved from "Dimensionless" and "submenu" to "Concentration"

Fix urls in plugin update_checker logs

[ci skip]

Cloudwatch dimension_values add dimension filter.

Adding Timeticks unit

(prometheus) support POST for query and query_range

Reported in #11176