Grow the open ledger expected transactions quickly (RIPD-1630):

* When increasing the expected ledger size, add on an extra 20%.
* When decreasing the expected ledger size, take the minimum of the
  validated ledger size or the old expected size, and subract another 50%.
* Update fee escalation documentation.
* Refactor the FeeMetrics object to use values from Setup

cfg/rippled-example.cfg

@@ -538,6 +538,20 @@
 #       into the ledger at the minimum required fee before the required
 #       fee escalates. Default: no maximum.
 #
+#   normal_consensus_increase_percent = <number>
+#
+#       (Optional) When the ledger has more transactions than "expected",
+#       and performance is humming along nicely, the expected ledger size
+#       is updated to the previous ledger size plus this percentage.
+#       Default: 20
+#
+#   slow_consensus_decrease_percent = <number>
+#
+#       (Optional) When consensus takes longer than appropriate, the
+#       expected ledger size is updated to the minimum of the previous
+#       ledger size or the "expected" ledger size minus this percentage.
+#       Default: 50
+#
 #   maximum_txn_per_account = <number>
 #
 #       Maximum number of transactions that one account can have in the