Switch refcount tracking from lists to AVL-trees.

With large number of tracked references list searches under the lock
become too expensive, creating enormous lock contention.

On my tests with ZFS_DEBUG enabled this increases write throughput
with 32KB blocks from ~1.2GB/s to ~7.5GB/s.

Reviewed-by: Brian Atkinson <[email protected]>
Reviewed-by: Brian Behlendorf <[email protected]>
Signed-off-by: Alexander Motin <[email protected]>
Sponsored by:	iXsystems, Inc.
Closes #14970 

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