Add sane fallback for fee estimation

- Always respect GetRequiredFee for wallet txs
- Add sane fallback for fee estimation
- SQUASHME: Fix rpc tests that assumed fallback to minRelayTxFee

Add new commandline option "-fallbackfee" to use when fee estimation does not have sufficient data.

Github-Pull: #7296
Rebased-From: 995b9f3 e420a1b bebe58b

qa/rpc-tests/fundrawtransaction.py

@@ -30,6 +30,11 @@ class RawTransactionsTest(BitcoinTestFramework):
         print "Mining blocks..."
 
         min_relay_tx_fee = self.nodes[0].getnetworkinfo()['relayfee']
+        # This test is not meant to test fee estimation and we'd like
+        # to be sure all txs are sent at a consistent desired feerate
+        for node in self.nodes:
+            node.settxfee(min_relay_tx_fee)
+
         # if the fee's positive delta is higher than this value tests will fail,
         # neg. delta always fail the tests.
         # The size of the signature of every input may be at most 2 bytes larger
@@ -447,6 +452,10 @@ class RawTransactionsTest(BitcoinTestFramework):
         wait_bitcoinds()
 
         self.nodes = start_nodes(4, self.options.tmpdir)
+        # This test is not meant to test fee estimation and we'd like
+        # to be sure all txs are sent at a consistent desired feerate
+        for node in self.nodes:
+            node.settxfee(min_relay_tx_fee)
 
         connect_nodes_bi(self.nodes,0,1)
         connect_nodes_bi(self.nodes,1,2)

qa/rpc-tests/mempool_limit.py

@@ -33,7 +33,9 @@ class MempoolLimitTest(BitcoinTestFramework):
         inputs = [{ "txid" : us0["txid"], "vout" : us0["vout"]}]
         outputs = {self.nodes[0].getnewaddress() : 0.0001}
         tx = self.nodes[0].createrawtransaction(inputs, outputs)
+        self.nodes[0].settxfee(self.relayfee) # specifically fund this tx with low fee
         txF = self.nodes[0].fundrawtransaction(tx)
+        self.nodes[0].settxfee(0) # return to automatic fee selection
         txFS = self.nodes[0].signrawtransaction(txF['hex'])
         txid = self.nodes[0].sendrawtransaction(txFS['hex'])
         self.nodes[0].lockunspent(True, [us0])