Use correct SQL_BIGINT constant for int8

SQL_BIGINT has been exported by DBI since version 1.54, so bump the
version requirement and use that instead of the incorrect SQL_DOUBLE.
Bump version to DBI to 1.54 everywhere. Credit recent change.
Fix assertion when binding array columns on debug perls >= 5.16
Bump Perl requirement to 5.8.4. May go up further, but let's put this stake in the ground.
Make some tests AUTHOR_TESTING, not RELEASE_TESTING, as we really don't want anyone else to run them.
Adjust test to use 3 digit exponential values (CPAN bug #59449)