Pair: Do not directly invoke toString
Pair: Do not directly invoke toString

Pair.toString was directly invoking toString on the member variables. This will
cause an NPE if either of the variables was null. Instead of directly invoking
toString allow java's string concatination implementation to correctly handle
variables.

Use Objects.equals to simplify contains implementation

VoltDB/voltdb,VoltDB/voltdb,VoltDB/voltdb,VoltDB/voltdb,VoltDB/voltdb,VoltDB/voltdb,VoltDB/voltdb