media_index.c: Refactored so it doesn't cache the index

Testing revealed that the cache added no benefit but that it could
consume excessive memory.

Two new index related functions were created:
ast_sounds_get_index_for_file() and ast_media_index_update_for_file()
which restrict index updating to specific sound files.

The original ast_sounds_get_index() and ast_media_index_update()
calls are still available but since they no longer cache the results
internally, developers should re-use an index they may already have
instead of calling ast_sounds_get_index() repeatedly.  If information
for only a single file is needed, ast_sounds_get_index_for_file()
should be called instead of ast_sounds_get_index().

The media_index directory scan code was elimininated in favor of
using the existing ast_file_read_dirs() function.

Since there's no more cache, ast_sounds_index_init now only
registers the sounds cli commands instead of generating the
initial index and subscribing to stasis format register/unregister
messages.

ast_sounds_reindex() is now a no-op but left for backwards
compatibility.

loader.c no longer registers "sounds" as a special reload target.

Both the sounds cli commands and the sounds ari resources were
refactored to only call ast_sounds_get_index() once per invocation
and to use ast_sounds_get_index_for_file() when a specific sound
file is requested.

Change-Id: I1cef327ba1b0648d85d218b70ce469ad07f4aa8d

main/loader.c

@@ -56,7 +56,6 @@ ASTERISK_FILE_VERSION(__FILE__, "$Revision$")
 #include "asterisk/vector.h"
 #include "asterisk/app.h"
 #include "asterisk/test.h"
-#include "asterisk/sounds_index.h"
 #include "asterisk/cli.h"
 
 #include <dlfcn.h>
@@ -376,7 +375,6 @@ static struct reload_classes {
 	{ "logger",      logger_reload },
 	{ "manager",     reload_manager },
 	{ "plc",         ast_plc_reload },
-	{ "sounds",      ast_sounds_reindex },
 	{ "udptl",       ast_udptl_reload },
 	{ NULL,          NULL }
 };