ENG-4213: Hopefully fixes the "I can't drain" issue.
Proc and Topo invocations didn't have unique handles per procedure which meant they might corrupt the callback tracking map.
ENG-4213: Make sure proc call handles are unique with an assertion.
Per-connection that is.
Merge pull request #346 from VoltDB/ENG-4213-triplehotfix

ENG-4213: Hopefully fixes the "I can't drain" issue.