NIFI-2323: Fixed Oracle timestamp handling in DB fetch processors

[mattyb149]

I tested this on Oracle 11g XE with tables containing TIMESTAMP and DATE fields. The state is now stored as timestamps (dates have a time-of-day of zero/midnight)

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[mattyb149]

This probably breaks backwards compatibility if the processor has already stored a Date (no time-of-day) into the StateMap. To preserve backwards compatibility, we should check if the value is just a Date and use the "date" coercion instead of the "timestamp" coercion. If that succeeds, then the next maximum value observed should be stored as a full timestamp and the "date" coercion should no longer be executed

[markap14]

This looks good to me. Code is clean and makes sense. Contrib-check passes with all unit tests. +1 merged to master.