Add a limit to key length, check key and value length on client side.

Currently there is no limit on key length and there should be. It should be a parameter in HTableDescriptor since the row key length needs to be considered in addition to the column key.

It should be trivial to add, since HTD can be upgraded without requiring a migration.

Checking of the key length (and the value length) should be done on the client side as it will fail early rather than once the request is sent to the server.

This means that a BatchUpdate needs a reference to either the HTable or to the HTD. It can be a transient reference so that the HTable (or HTD) need not be serialized/deserialized.