Add option for controlling whether to produce tombstone records on DELETE operations

Debezium cannot possibly predict how users are going to use the data on the topic, so forcing tombstones without any config parameter seems to be very arbitrary.
Some may not want compacting to happen (like in our case), therefore the connector should allow the default (with tombstone) behaviour to be changed so that the null record (tombstone) won't be produced.
Please see PR with the proposed solution: https://github.com/debezium/debezium/pull/421
Discussed with gunnar.morling