Transaction queue and fee escalation (RIPD-598):

The first few transactions are added to the open ledger at
the base fee (ie. 10 drops).  Once enough transactions are
added, the required fee will jump dramatically. If additional
transactions are added, the fee will grow exponentially.

Transactions that don't have a high enough fee to be applied to
the ledger are added to the queue in order from highest fee to
lowest. Whenever a new ledger is accepted as validated, transactions
are first applied from the queue to the open ledger in fee order
until either all transactions are applied or the fee again jumps
too high for the remaining transactions.

Current implementation is restricted to one transaction in the
queue per account. Some groundwork has been laid to expand in
the future.

Note that this fee logic escalates independently of the load-based
fee logic (ie. LoadFeeTrack). Submitted transactions must meet
the load fee to be considered for the queue, and must meet both
fees to be put into open ledger.

doc/rippled-example.cfg

@@ -373,6 +373,30 @@
 #       connections to or from this server.
 #
 #
+# [transaction_queue] EXPERIMENTAL
+#
+#   This section is EXPERIMENTAL, and should not be
+#   present for production configuration settings.
+#
+#   A set of key/value pair parameters to tune the performance of the
+#   transaction queue.
+#
+#   ledgers_in_queue = <number>
+#
+#       The queue will be limited to this <number> of average ledgers'
+#       worth of transactions. If the queue fills up, the transactions
+#       with the lowest fees will be dropped from the queue any time a
+#       transaction with a higher fee level is added. Default: 20.
+#
+#   retry_sequence_percent = <number>
+#
+#       If a client resubmits a transaction, the new transaction's fee
+#       must be more than <number> percent higher than the original
+#       transaction's fee, or meet the current open ledger fee to be
+#       considered. Default: 125.
+#
+#
+#
 #-------------------------------------------------------------------------------
 #
 # 3. Ripple Protocol
@@ -667,7 +691,7 @@
 #
 # [debug_logfile]
 #
-#   Specifies were a debug logfile is kept. By default, no debug log is kept.
+#   Specifies where a debug logfile is kept. By default, no debug log is kept.
 #   Unless absolute, the path is relative the directory containing this file.
 #
 #   Example: debug.log