Use MutableByteArray as buffers, add manual keepAlive

[jorisdral]

Before, on d38d85e

Low-level API benchmark
Total I/O ops: 262144
Elapsed time:  1.146791772s
IOPS:          228589

High-level API benchmark
Total I/O ops: 262144
Elapsed time:  1.310599797s
IOPS:          200018

After, on bbeb811

Low-level API benchmark
Total I/O ops: 262144
Elapsed time:  1.157516626s
IOPS:          226471

High-level API benchmark
Total I/O ops: 262144
Elapsed time:  1.315785114s
IOPS:          199230

Virtually the same

[dcoutts]

I suggest we do check the bytearrays are pinned and re-run the benchmark to be sure that doesn't have a significant impact.

[jorisdral]

The latest commit (bb4b65c) added a guard that checks that byte arrays are pinned. There's probably a slight performance decrease, but not a very large one:

Low-level API benchmark
Total I/O ops: 262144
Elapsed time:  1.163442451s
IOPS:          225318

High-level API benchmark
Total I/O ops: 262144
Elapsed time:  1.325703812s
IOPS:          197739

The change only affects the high-level benchmark