Driver loading previously was very dependent on what code clojure
compiled first. If the Oracle driver was loaded before the plugin
directory was examined, it would result in the Oracle driver never
being loaded. This is due to clojure code only being loaded once when
required and the driver loading code being free floating code that is
only evaled when the namespace is loaded.

This commit switches that code to be in an -init-driver function that
is invoked when the driver is required. This ensures that regardless
of code loading order, the driver will be available if it's present.

This commit also includes code to fail the build if requested to test
on an engine when that engine's driver is not available. Previously if
you set ENGINES to "h2,oracle" but had no Oracle driver in the plugins
directory, the test would succeed, but only test against H2.

This unifies the code and the environment variables for specifying
test database connection parameters across all supported
databases. Existing databases that supported environment variables for
tests have been refactored to use the new environment variable
names. All of the connection environment variables are named similar
to:

MB_<DBNAME>_TEST_<PARAM>

For user on MySQL, the variable would be named:

MB_MYSQL_TEST_USER

+ no longer returning datetimes as longs

It was expecting an exact number of keep-allive packets, now it just
checks that there are at least two.

Fixes #5016 and adds tests to all the drivers with ssh tunnels that
implement their own can-connect?

* api/dataset/:id
* card/:id/query