This commit adds a new keypair to the fields map called
"default_dimension_option". For datetimes, this is grouping by day,
for binning this is the "default" binning strategy.

This hadn't been updated to allow for the two binning strategies that
we now support.

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

This commit also changes from an aliased projection, which does not
work on all SQL databases to a repeated expression. The equation now
appears in the SELECT, GROUP BY and ORDER BY. This whould work across
SQL databases.

In MBQL you can now specify a bin-width binning strategy with a
specified bin-width and the BinnedField will bin using that
information.

This commit also changes the existing behavior of the "default"
binning strategy. When default is specified (for now) this uses the
"num-bins" strategy with the globally set default number of
bins (specified via setting). Specifying num-bins or bin-width
requires an extra parameter specifying the number of bins or the bin
width. Only default uses the global setting.

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