To get rid of issues involving date/time when testing.
Also, makes it possible to run mysql integration tests for both
grafana config db and tsdb at the same time using
GRAFANA_TEST_DB=mysql go test ./pkg/...
Use the ConvertSqlTimeColumnToEpochMs function to convert any native
datetime data type or epoch time (millisecond precision).
Refactored mysql implementation to make it more similar to postgres
and mssql implementations.
Added $__timeEpoch macro function with same implementation as $__time.
Added possibility to use a time column named time in addition to
the currectly supported time_sec.
Additional tests and update of existing.
Added test dashboard.
