DERBY-5840: Clean up compiler warnings introduced by using Java 5 language features

Store lists of implementation classes in a list instead of an array,
since arrays cannot have generic types. This silences an unchecked
conversion warning and allows us to remove a SuppressWarnings
annotation.

git-svn-id: https://svn.apache.org/repos/asf/db/derby/code/trunk@1486826 13f79535-47bb-0310-9956-ffa450edef68