Replace median fee rate with feerate percentiles

Removes medianfeerate result from getblockstats.
Adds feerate_percentiles which give the feerate of the 10th, 25th, 50th,
75th, and 90th percentile weight unit in the block.

test/functional/rpc_getblockstats.py

@@ -27,7 +27,7 @@ class GetblockstatsTest(BitcoinTestFramework):
         'maxfee',
         'maxfeerate',
         'medianfee',
-        'medianfeerate',
+        'feerate_percentiles',
         'minfee',
         'minfeerate',
         'totalfee',