The current xxHash implementation hashes the whole packet

[jakubcech]

Description

The change introduced in #1326 hashes not only the transaction trytes, but also additional packet info like the request bytes. This needs to be changed back so that only the tx trytes are in the digest.

Motivation

The current implementation is incorrect and creates a lot of misses on the cache.

Whole packet hash:

iri/src/main/java/com/iota/iri/network/Node.java

Line 296 in 5c2eacb

long digest = getBytesDigest(receivedData);

Only TX hash:

iri/src/main/java/com/iota/iri/network/Node.java

Line 516 in 5c2eacb

long digest = getBytesDigest(transactionViewModel.getBytes());