Add /crawl cgi request feature to peer protocol (RIPD-729):

This adds support for a cgi /crawl request, issued over HTTPS to the configured
peer protocol port. The response to the request is a JSON object containing
the node public key, type, and IP address of each directly connected neighbor.
The IP address is suppressed unless the neighbor has requested its address
to be revealed by adding "Crawl: public" to its HTTP headers. This field is
currently set by the peer_private option in the rippled.cfg file.

doc/rippled-example.cfg

@@ -688,9 +688,7 @@
 #       The 'temp_db' configures a look-aside cache for high volume storage
 #           which doesn't necessarily persist between server launches. This
 #           is an optional configuration parameter. If it is left out then
-#           no look-aside database is created or used. Use of temp_db may
-#           improve performance in some environments, but should be tested
-#           with and without to determine if beneficial.
+#           no look-aside database is created or used.
 #
 #       The 'import_db' is used with the '--import' command line option to
 #           migrate the specified database into the current database given