[Java] headroom does not take into account reservation

[asfimport]

It looks like BufferAllocator#getHeadroom() does not take into account current allocator reservation. For example if a parent allocator has a limit of 10, and a child allocator is created with a reservation of 6, the headroom of the parent is 4, and the current headroom will report 4, but the actual headroom is 10 because the child has no memory used and so can use the full extend of its reservation on top of what the parent allocator can allocate

Reporter: Laurent Goujon / @laurentgo
Assignee: Laurent Goujon / @laurentgo

PRs and other links:

• GitHub Pull Request #2434

_{Note: This issue was originally created as ARROW-3061. Please see the migration documentation for further details.}