DRILL-4246: Fix Allocator concurrency bug and improve error detection
- Rename the internal DrillBuf field to udle to better express its purpose.
- Rename AllocatorManager to AllocationManager to better express its purpose.
- Address situation where dangling ledger could be transferred into while it was being released released by protecting association and release inside the AllocationManager.
- Add allocator assertions to ensure allocator operations are done while the allocator is open.
- Simplify AllocationManager locking model.
- Exclude HDFS reference to netty-all
- Improve debugging messages for allocators (and fix debug message bugs)
This closes #323.
