[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.

test/functional/p2p_opportunistic_1p1c.py

@@ -13,10 +13,12 @@ import time
 from test_framework.blocktools import MAX_STANDARD_TX_WEIGHT
 from test_framework.mempool_util import (
     create_large_orphan,
+    DEFAULT_MIN_RELAY_TX_FEE,
     fill_mempool,
 )
 from test_framework.messages import (
     CInv,
+    COIN,
     COutPoint,
     CTransaction,
     CTxIn,
@@ -79,13 +81,13 @@ class PackageRelayTest(BitcoinTestFramework):
         ]]
 
     def create_tx_below_mempoolminfee(self, wallet, utxo_to_spend=None):
-        """Create a 1-input 1sat/vB transaction using a confirmed UTXO. Decrement and use
+        """Create a 1-input 0.1sat/vB transaction using a confirmed UTXO. Decrement and use
         self.sequence so that subsequent calls to this function result in unique transactions."""
 
         self.sequence -= 1
-        assert_greater_than(self.nodes[0].getmempoolinfo()["mempoolminfee"], FEERATE_1SAT_VB)
+        assert_greater_than(self.nodes[0].getmempoolinfo()["mempoolminfee"], Decimal(DEFAULT_MIN_RELAY_TX_FEE) / COIN)
 
-        return wallet.create_self_transfer(fee_rate=FEERATE_1SAT_VB, sequence=self.sequence, utxo_to_spend=utxo_to_spend, confirmed_only=True)
+        return wallet.create_self_transfer(fee_rate=Decimal(DEFAULT_MIN_RELAY_TX_FEE) / COIN, sequence=self.sequence, utxo_to_spend=utxo_to_spend, confirmed_only=True)
 
     @cleanup
     def test_basic_child_then_parent(self):