[Java] Remove Netty dependency for BufferAllocator and ReferenceManager

[asfimport]

With previous work (ARROW-7329 and ARROW-7505), Netty based allocation is only one of the possible implementations. So we need to revise BufferAllocator and ReferenceManager, to make them general, and independent of Netty libraries.

Reporter: Liya Fan / @liyafan82
Assignee: Liya Fan / @liyafan82

Related issues:

• [Java] Remove Netty references from ArrowBuf and move Allocator out of vector package (is a child of)

PRs and other links:

• GitHub Pull Request #6487

_{Note: This issue was originally created as ARROW-7935. Please see the migration documentation for further details.}

[asfimport]

Micah Kornfield / @emkornfield:
Issue resolved by pull request 6487
#6487