Add validation to make sure the following two configs are not null:

• `commit.log.relocation.dir`
• `offset.backing.store.dir`

Theses two configs are required by OffsetWriter and QueuingProcessor, but we can't set default values for them since they correspond to directories users are supposed to create by themselves.

For some context, see comments in https://github.com/debezium/debezium-incubator/pull/163