ref #9024

If the template variable query has two columns named __text and __value
then return a list of key values. The value is used when the variable is
interpolated in the query. Allows mapping of texts to ids.

Fixes #8855

and a fix for when selecting the same value twice in
a template variable if it is not multi select. It threw an
exception on the second selection

fix: templating issue with data source variables, can now be used right away in other query variables and panels (without page reload), fixes #7510

fix: MySQL/Postgress max_idle_conn default was wrongly set to zero, which does not mean unlimited but zero, which in practice disables connection pooling, not good. now max idle conn is set to golang's default which is 2, fixes #8513

Includes integration test for most column types. Fixes #8918

* $__timeFrom() -> Returns the dashboard 'from' suitable for use querying
  against a MySQL TIMESTAMP field.
* $__timeTo() -> Returns the dashboard 'to' suitable for use querying
  against a MySQL TIMESTAMP field.
* $__unixEpochFiler(column) -> If you store timestamps as UNIX epoch's,
  this builds: column > 'from' AND column < 'to'
* $__unixEpochFrom() -> Returns the dashboard 'from' suitable for use querying
  against a MySQL integer field (UNIX epochs)
* $__unixEpochTo() -> Returns the dashboard 'to' suitable for use querying
  against a MySQL integer field (UNIX epochs)