mu-server: implement temp-file optimization

It can be faster to feed big mu -> mu4e data, such as contacts are
message headers through a temp-file instead directly though stdout;
implement this, and add the server parameter --allow-temp-file.

Implement this the "contacts" and "find" commands.

mu/mu-cmd-server.cc

@@ -117,7 +117,10 @@ Mu::mu_cmd_server(const Mu::Options& opts) try {
 	if (!store)
 		return Err(store.error());
 
-	Server server{*store, output_sexp_stdout};
+	Server::Options sopts{};
+	sopts.allow_temp_file = opts.server.allow_temp_file;
+
+	Server server{*store, sopts, output_sexp_stdout};
 	mu_message("created server with store @ {}; maildir @ {}; debug-mode {};"
 		  "readline: {}",
 		   store->path(), store->root_maildir(),

mu/mu-options.cc

@@ -498,6 +498,10 @@ sub_server(CLI::App& sub, Options& opts)
 	sub.add_option("--eval", opts.server.eval,
 		       "Evaluate mu server expression")
 		->excludes("--commands");
+	sub.add_flag("--allow-temp-file", opts.server.allow_temp_file,
+		     "Allow for the temp-file optimization")
+		->excludes("--commands");
+
 }
 
 static void

mu/mu-options.hh

@@ -222,6 +222,7 @@ struct Options {
 	struct Server {
 		bool		commands;		/**< dump docs for commands */
 		std::string	eval;			/**< command to evaluate */
+		bool            allow_temp_file;	/**< temp-file optimization allowed? */
 	} server;
 
 	/*