Backport to JDK 6

Replaced all uses of ju.Objects.hashCode and ju.Objects.equals with a
copy of their implementations inlined into the code.

Also changed source/target for compiler plugin.

To test, I compiled my own avaje launchagent against 6, and changed to
use that in the pom, compiled/tested the whole project using JDK 7,
then ran mvn surefire:test using JDK 6 - running surefire:test ensures
that mvn doesn't try to recompile everything against 6, since that's not
possible because of some of the delegate classes having delegate methods
to JDK 7 jdbc classes.
Merge pull request #129 from jroper/java16-compat

Backport to JDK 6