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

Remove SuppressWarnings annotation in DiskHashtable and use strongly
typed variables instead of an overloaded Object variable to prevent
the warnings.

git-svn-id: https://svn.apache.org/repos/asf/db/derby/code/trunk@1486780 13f79535-47bb-0310-9956-ffa450edef68