[miner] lower default -blockmintxfee to 1sat/kvB

Back when we implemented coin age priority as a miner policy, miners
mempools might admit transactions paying very low fees, but then want to
set a higher fee for block inclusion. However, since coin age priority
was removed in v0.15, the block assembly policy is solely based on fees,
so we do not need to apply minimum feerate rules in multiple places. In
fact, the block assembly policy ignoring transactions that are added to
the mempool is likely undesirable as we waste resources accepting and
storing this transaction.

Instead, rely on mempool policy to enforce a minimum entry feerate to
the mempool (minrelaytxfee). Set the minimum block feerate to the
minimum non-zero amount (1sat/kvB) so it collects everything it finds in
mempool into the block.

test/functional/mining_basic.py

@@ -55,7 +55,7 @@ MAX_FUTURE_BLOCK_TIME = 2 * 3600
 MAX_TIMEWARP = 600
 VERSIONBITS_TOP_BITS = 0x20000000
 VERSIONBITS_DEPLOYMENT_TESTDUMMY_BIT = 28
-DEFAULT_BLOCK_MIN_TX_FEE = 1000  # default `-blockmintxfee` setting [sat/kvB]
+DEFAULT_BLOCK_MIN_TX_FEE = 1 # default `-blockmintxfee` setting [sat/kvB]
 
 class MiningTest(BitcoinTestFramework):
     def set_test_params(self):
@@ -144,7 +144,7 @@ class MiningTest(BitcoinTestFramework):
         node = self.nodes[0]
 
         # test default (no parameter), zero and a bunch of arbitrary blockmintxfee rates [sat/kvB]
-        for blockmintxfee_sat_kvb in (DEFAULT_BLOCK_MIN_TX_FEE, 0, 1, 5, 10, 50, 100, 500, 2500, 5000, 21000, 333333, 2500000):
+        for blockmintxfee_sat_kvb in (DEFAULT_BLOCK_MIN_TX_FEE, 0, 5, 10, 50, 100, 500, 1000, 2500, 5000, 21000, 333333, 2500000):
             blockmintxfee_btc_kvb = blockmintxfee_sat_kvb / Decimal(COIN)
             if blockmintxfee_sat_kvb == DEFAULT_BLOCK_MIN_TX_FEE:
                 self.log.info(f"-> Default -blockmintxfee setting ({blockmintxfee_sat_kvb} sat/kvB)...")