[policy] lower default minrelaytxfee and incrementalrelayfee to 100sat/kvB

Let's say an attacker wants to use/exhaust the network's bandwidth, and
has the choice between renting resources from a commercial provider and
getting the network to "spam" itself it by sending unconfirmed
transactions. We'd like the latter to be more expensive than the former.

The bandwidth for relaying a transaction across the network is roughly
its serialized size (plus relay overhead) x number of nodes. A 1000vB
transaction is 1000-4000B serialized. With 100k nodes, that's 0.1-0.4GB
If the going rate for commercial services is 10c/GB, that's like 1-4c per kvB
of transaction data, so a 1000vB transaction should pay at least $0.04.

At a price of 120k USD/BTC, 100sat is about $0.12. This price allows us
to tolerate a large decrease in the conversion rate or increase in the
number of nodes.

Github-Pull: #33106
Rebased-From: 6da5de58ca

test/functional/test_framework/mempool_util.py

@@ -20,9 +20,9 @@ from .wallet import (
 
 ORPHAN_TX_EXPIRE_TIME = 1200
 # Default for -minrelaytxfee in sat/kvB
-DEFAULT_MIN_RELAY_TX_FEE = 1000
+DEFAULT_MIN_RELAY_TX_FEE = 100
 # Default for -incrementalrelayfee in sat/kvB
-DEFAULT_INCREMENTAL_RELAY_FEE = 1000
+DEFAULT_INCREMENTAL_RELAY_FEE = 100
 
 def assert_mempool_contents(test_framework, node, expected=None, sync=True):
     """Assert that all transactions in expected are in the mempool,