[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_ibd_txrelay.py

@@ -28,8 +28,8 @@ from test_framework.p2p import (
 )
 from test_framework.test_framework import BitcoinTestFramework
 
-MAX_FEE_FILTER = Decimal(9170997) / COIN
-NORMAL_FEE_FILTER = Decimal(100) / COIN
+MAX_FEE_FILTER = Decimal(9936506) / COIN
+NORMAL_FEE_FILTER = Decimal(10) / COIN
 
 
 class P2PIBDTxRelayTest(BitcoinTestFramework):
@@ -37,8 +37,8 @@ class P2PIBDTxRelayTest(BitcoinTestFramework):
         self.setup_clean_chain = True
         self.num_nodes = 2
         self.extra_args = [
-            ["-minrelaytxfee={}".format(NORMAL_FEE_FILTER)],
-            ["-minrelaytxfee={}".format(NORMAL_FEE_FILTER)],
+            ["-minrelaytxfee={:.8f}".format(NORMAL_FEE_FILTER)],
+            ["-minrelaytxfee={:.8f}".format(NORMAL_FEE_FILTER)],
         ]
 
     def run_test(self):