Recover open ledger transactions to the queue (RIPD-1530):

* If the transaction can't be queued, recover to the open ledger once,
  and drop it on the next attempt.
* New result codes for transactions that can not queue.
* Add minimum queue size.
* Remove the obsolete and incorrect SF_RETRY flag.
* fix #2215

doc/rippled-example.cfg

@@ -475,6 +475,12 @@
 #       time a transaction with a higher fee level is added.
 #       Default: 20.
 #
+#   minimum_queue_size = <number>
+#
+#       The queue will always be able to hold at least this <number> of
+#       transactions, regardless of recent ledger sizes or the value of
+#       ledgers_in_queue. Default: 2000.
+#
 #   retry_sequence_percent = <number>
 #
 #       If a client replaces a transaction in the queue (same sequence