[kv] Use JDK9 Arrays#compare() to check bytes prefix for PrefixLookup

[swuferhong]

Purpose

Linked issue: #271

Currently, RocksDBKv#isPrefixEquals() very inefficient to compare arrays byte by byte. Java9 provides a more efficient way Arrays.compare(compare(byte[] a, int aFromIndex, int aToIndex, byte[] b, int bFromIndex, int bToIndex)), underlying it leverages SIMD instructions.

As it is only supported since Java9, In this pr, we implement it based on the running jdk env (the method can get the running JDK environment, if Java9 use the support Arrays.compare() method).

Tests

API and Format

Documentation

[wuchong]

@swuferhong I adjusted the implementation a bit.

1. extract the JDK9 compare logic from RocksDBKv into a common util so that it can be reused by others in the future.
2. Use Arrays.equals(byte[], int, int, byte[], int, int) instead, because we only need to compare the prefix equality.
3. Avoid to new PrefixComparer for each RocksDBKv instance. In the final, it's just a util method.

[wuchong]

LGTM