Change fee escalation algorithms (RIPD-1177):

* Minimum factor 256*500, don't multiply by base fee
* Change autofill fee behavior to pay the open ledger fee.
** Experimental options: x-assume-tx - assume <int> more transactions in
   the open queue when computing escalated fee, x-queue-okay - if true
   and escalated fee is over limit, try with load fee.
* Port of 75af4ed.

src/ripple/rpc/tests/Subscribe.test.cpp

@@ -313,7 +313,7 @@ public:
                         jv[jss::flags] ==
                             (vfFullyCanonicalSig | STValidation::kFullFlag) &&
                         jv[jss::full] == true &&
-                        jv[jss::load_fee] == 256000 &&
+                        !jv.isMember(jss::load_fee) &&
                         jv[jss::signature] &&
                         jv[jss::signing_time];
                 }));