@ahrens I tried to implement the metaslab weight adjustment when taking chunks 
out of the metaslab due to trimming. The case when the metaslab is loaded is 
easy. However, the unloaded case, where we only have the spacemap histogram, is 
not possible. Here's why: I can construct a "negative" histogram from the 
chunks being trimmed, that's easy. However, I can't simply subtract the values 
in the buckets of the negative histogram from the spacemap histogram. That's 
because without the spacemap loaded into a range tree, I can't know which free 
extents I need to reduce in size and consequently move them between histogram 
buckets. Put simply, without a range tree in hand, I can't know where the 
trimming extents came from.

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