Update the convolution code:

(1) to be multithreaded,
    (2) to work on multichannel audio and
    (3) to work on 48k and 44.1k audio.

    Allow multiple impulse response (IR) files with a new setting: "convolution_ir_files"
    When convolution starts, Shairport Sync will look for an IR file with a sample rate
    matching the input (44.1k or 48k) and channel count.
    If one can't be found, it will look for a single-channel IR file with the same rate.
    It will always choose the first match in the file list supplied "convolution_ir_files".

    Allow multithreading -- use the "convolution_thread_pool_size" to set the number of threads to use.

    Deprecate "convolution" -- use "convolution_enabled" instead.
    Deprecate "convolution_max_length" -- use "convolution_max_length_in_seconds" instead.
    Deprecate "convolution_ir_file" -- use "convolution_ir_files" instead.
    Update corresponding D-Bus methods and properties.

Update the loudness code to work on 48k and well as 44.1k audio and with multichannel audio.

    Deprecate "loudness" -- use "loudness_enabled" instead.
    Update corresponding D-Bus methods and properties.

Fix a deprecated FFmpeg warning.

Update HiFi-LoFi FFT convolver to latest available.

rtsp.c

@@ -99,6 +99,11 @@
 #endif
 #endif
 
+#ifdef CONFIG_CONVOLUTION
+#include "FFTConvolver/convolver.h"
+#endif
+
+
 #ifdef CONFIG_DBUS_INTERFACE
 #include "dbus-service.h"
 #endif
@@ -2016,6 +2021,10 @@ void handle_setrateanchori(rtsp_conn_info *conn, rtsp_message *req, rtsp_message
         debug(2, "Connection %d: SETRATEANCHORI Pause playing.", conn->connection_number);
         conn->ap2_play_enabled = 0;
         activity_monitor_signify_activity(0);
+#ifdef CONFIG_CONVOLUTION
+        // convolver_clear_state();
+#endif
+
         // reset_anchor_info(conn);
 #ifdef CONFIG_METADATA
         send_ssnc_metadata('paus', NULL, 0, 1); // pause -- contains cancellation points