mu-server: use strings, not sexps object (optimization)

When passing messages to mu, often we got a (parsed from string)
message-sexp from the message document; then appended some more
properties ("build_message_sexp").

Instead, we can do it in terms of the strings; this is _a little_
inelegant, but also much faster; compare:

(base)
[mu4e] Found 500 matching messages; 0 hidden; search: 1298.0 ms (2.60 ms/msg); render: 642.1 ms (1.28 ms/msg)

(with temp-file optimization (earlier commit)
[mu4e] Found 500 matching messages; 0 hidden; search: 1152.7 ms (2.31 ms/msg); render: 270.1 ms (0.54 ms/msg)

(with temp file optimize _and_ the string opt (this commit)
[mu4e] Found 500 matching messages; 0 hidden; search: 266.0 ms (0.53 ms/msg); render: 199.7 ms (0.40 ms/msg)

lib/mu-server.hh

@@ -36,19 +36,16 @@ class Server {
 public:
 	enum struct OutputFlags {
 		None	  = 0,
-		SplitList = 1 << 0,
-		/**< insert newlines between list items */
-		Flush	  = 1 << 1,
-		/**< flush output buffer after */
+		Flush	  = 1 << 0, /**< flush output buffer after */
 	};
 
 	/**
 	 * Prototype for output function
 	 *
-	 * @param sexp an s-expression
+	 * @param str a string
 	 * @param flags flags that influence the behavior
 	 */
-	using Output = std::function<void(const Sexp& sexp, OutputFlags flags)>;
+	using Output = std::function<void(const std::string& str, OutputFlags flags)>;
 
 	struct Options {
 		bool allow_temp_file; /**< temp file optimization allowed? */