Restrict direct access to `sun.misc.Unsafe`

[laurentgo]

Describe the enhancement requested

sun.misc.Unsafe is a Java internal class only accessible to classes loaded of the boot classloader, unless one uses reflection to bypass this restriction.

org.apache.arrow.memory.util.MemoryUtil makes it available as a public field to any java classes which is kind of opening a pandora box. As a first step towards switching from sun.misc.Unsafe to safer memory access methods (which may become a requirement at some point as discussed in JEP 471 ), remove direct access to sun.misc.Unsafe instance.

Component(s)

Java

[danepitkin]

Issue resolved by pull request 43480
#43480