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.

rtp.c

@@ -45,6 +45,7 @@
 #include <sys/types.h>
 #include <time.h>
 #include <unistd.h>
+
 #ifdef CONFIG_AIRPLAY_2
 // #include "plist_xml_strings.h"
 #include "ptp-utilities.h"
@@ -56,6 +57,11 @@
 #include <sodium.h>
 #endif
 
+#ifdef CONFIG_CONVOLUTION
+#include "FFTConvolver/convolver.h"
+#endif
+
+
 struct Nvll {
   char *name;
   double value;
@@ -2484,6 +2490,9 @@ void *rtp_buffered_audio_processor(void *arg) {
       //     0; // This may be set to 1 by a flush, so don't zero it during start.
       packets_played_in_this_sequence = 0;
       new_buffer_needed = 0;
+#ifdef CONFIG_CONVOLUTION
+      convolver_clear_state();
+#endif
     }
 
     if ((play_enabled == 0) && (conn->ap2_play_enabled != 0)) {