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

Remove the @SuppressWarnings("unchecked") annotation from
BackingStoreHashtable and introduce a specialized sub-class of
ArrayList to make the casts type safe.

git-svn-id: https://svn.apache.org/repos/asf/db/derby/code/trunk@1486782 13f79535-47bb-0310-9956-ffa450edef68