Accept batch read response without key digest

[jhecking]

The current batch read response protocol sends back the digest for every record. This information was used to verify that the batch index (also sent back for every record) pointed to the correct key entry. Because the digest is not strictly necessary, we plan to eventually remove it from the batch response protocol.

E.g. Java client change (CLIENT-1119): aerospike/aerospike-client-java@27a855a