Configurable fetch depth.

doc/rippled-example.cfg

@@ -494,6 +494,20 @@
 #
 #
 #
+# [fetch_depth]
+#
+#   The number of past ledgers to serve to other peers that request historical
+#   ledger data (or "full" for no limit).
+#
+#   Servers that require low latency and high local performance may wish to
+#   restrict the historical ledgers they are willing to serve. Setting this
+#   below 32 can harm network stability as servers require easy access to
+#   recent history to stay in sync. Values below 128 are not recommended.
+#
+#   The default is: full
+#
+#
+#
 # [validation_seed]
 #
 #   To perform validation, this section should contain either a validation seed