Fix limit setting and add ServerImp tests (RIPD-1463,1458):

Add more test coverage for ServerHandlerImp.cpp. Ensure limit parameter
is propagated from parsed object to in-memory config.

Release Notes
-------------

This fixes a bug whereby the limit parameter on a port configuration was
ignored.

doc/rippled-example.cfg

@@ -172,6 +172,13 @@
 #       NOTE    If no ports support the peer protocol, rippled cannot
 #               receive incoming peer connections or become a superpeer.
 #
+#   limit = <number>
+#
+#       Optional. An integer value that will limit the number of connected
+#       clients that the port will accept. Once the limit is reached, new
+#       connections will be refused until other clients disconnect.
+#       Omit or set to 0 to allow unlimited numbers of clients.
+#
 #   user = <text>
 #   password = <text>
 #