Reject arrays as keys in DataStream API to avoid inconsistent hashing

This issue has been reported on the mailing list twice:

• http://apache-flink-user-mailing-list-archive.2336050.n4.nabble.com/Previously-working-job-fails-on-Flink-1-2-0-td11741.html
• http://apache-flink-user-mailing-list-archive.2336050.n4.nabble.com/Arrays-values-in-keyBy-td7530.html

The problem is the following: We are using just Key[].hashCode() to compute the hash when shuffling data. Java's default hashCode() implementation doesn't take the arrays contents into account, but the memory address.
This leads to different hash code on the sender and receiver side.
In Flink 1.1 this means that the data is shuffled randomly and not keyed, and in Flink 1.2 the keygroups code detect a violation of the hashing.

The proper fix of the problem would be to rely on Flink's TypeComparator class, which has a type-specific hashing function. But introducing this change would break compatibility with existing code.
I'll file a JIRA for the 2.0 changes for that fix.

For 1.2.1 and 1.3.0 we should at least reject arrays as keys.