Switch from sun.misc.Unsafe to java.nio.ByteBuffer for vectoer (de-)serialization (forwards and backwards compatible)

[alexklibisz]

Related Issue

#263

Changes

Migrating vector serialization functions from sun.misc.Unsafe to java.nio.ByteBuffer.

The new serialization and deserialization are both compatible with sun.misc.Unsafe, so users do not need to worry about re-indexing when they make an upgrade.

The motivations for this switch are:

1. sun.misc.Unsafe is deprecated and will be removed from the JVM at some point.
2. sun.misc.Unsafe requires extra security policies to install Elastiknn as a plugin. With ByteBuffer, we're able to remove these policies.
3. The ByteBuffer API is slightly nicer.

Micro benchmarks show the new functions are roughly as fast or faster across the board:

info] Benchmark                                                      Mode  Cnt          Score   Error  Units
[info] VectorSerializationBenchmarks.readFloats_ByteBuffer           thrpt         2121757.541          ops/s
[info] VectorSerializationBenchmarks.readFloats_Unsafe               thrpt         2194790.835          ops/s
[info] VectorSerializationBenchmarks.readInt_ByteBuffer              thrpt       111727965.417          ops/s
[info] VectorSerializationBenchmarks.readInt_Unsafe                  thrpt       165961369.448          ops/s
[info] VectorSerializationBenchmarks.readInts_ByteBuffer             thrpt         2207100.329          ops/s
[info] VectorSerializationBenchmarks.readInts_Unsafe                 thrpt         2136738.508          ops/s
[info] VectorSerializationBenchmarks.writeFloats_ByteBuffer          thrpt         2161012.290          ops/s
[info] VectorSerializationBenchmarks.writeFloats_Unsafe              thrpt         2189182.950          ops/s
[info] VectorSerializationBenchmarks.writeInt_ByteBuffer             thrpt       107505261.268          ops/s
[info] VectorSerializationBenchmarks.writeInt_Unsafe                 thrpt        27993290.467          ops/s
[info] VectorSerializationBenchmarks.writeIntsWithPrefix_ByteBuffer  thrpt         2170718.842          ops/s
[info] VectorSerializationBenchmarks.writeIntsWithPrefix_Unsafe      thrpt         2171472.999          ops/s
[info] VectorSerializationBenchmarks.writeInts_ByteBuffer            thrpt         2191712.505          ops/s
[info] VectorSerializationBenchmarks.writeInts_Unsafe                thrpt         2202876.831          ops/s

The only one that's slower is readInt, but that is only used in tests anyways.

The ann-benchmarks also look good:

Testing and Validation

Standard CI and JMH benchmarking. New unit tests also verify forwards/backwards compatibility with the old serialization format.