[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/mempool_ephemeral_dust.py

@@ -216,7 +216,7 @@ class EphemeralDustTest(BitcoinTestFramework):
 
         res = self.nodes[0].submitpackage([dusty_tx["hex"], sweep_tx["hex"]])
         assert_equal(res["package_msg"], "transaction failed")
-        assert_equal(res["tx-results"][dusty_tx["wtxid"]]["error"], "min relay fee not met, 0 < 147")
+        assert_equal(res["tx-results"][dusty_tx["wtxid"]]["error"], "min relay fee not met, 0 < 15")
 
         assert_equal(self.nodes[0].getrawmempool(), [])