Handle compression level in pg_receivewal for LZ4

The new option set of pg_receivewal introduced in 042a923 to control the
compression method makes it now easy to pass down various options,
including the compression level.  The change to be able to do is simple,
and requires one LZ4F_preferences_t fed to LZ4F_compressBegin().

Note that LZ4F_INIT_PREFERENCES could be used to initialize the contents
of LZ4F_preferences_t as required by LZ4, but this is only available
since v1.8.3.  memset()'ing its structure to 0 is enough.

Discussion: https://postgr.es/m/YlPQGNAAa04raObK@paquier.xyz

Author: michaelpq

src/bin/pg_basebackup/walmethods.c

@@ -165,6 +165,7 @@ dir_open_for_write(const char *pathname, const char *temp_suffix, size_t pad_to_
 	{
 		size_t		ctx_out;
 		size_t		header_size;
+		LZ4F_preferences_t prefs;
 
 		ctx_out = LZ4F_createCompressionContext(&ctx, LZ4F_VERSION);
 		if (LZ4F_isError(ctx_out))
@@ -177,8 +178,12 @@ dir_open_for_write(const char *pathname, const char *temp_suffix, size_t pad_to_
 		lz4bufsize = LZ4F_compressBound(LZ4_IN_SIZE, NULL);
 		lz4buf = pg_malloc0(lz4bufsize);
 
+		/* assign the compression level, default is 0 */
+		memset(&prefs, 0, sizeof(prefs));
+		prefs.compressionLevel = dir_data->compression_level;
+
 		/* add the header */
-		header_size = LZ4F_compressBegin(ctx, lz4buf, lz4bufsize, NULL);
+		header_size = LZ4F_compressBegin(ctx, lz4buf, lz4bufsize, &prefs);
 		if (LZ4F_isError(header_size))
 		{
 			dir_data->lasterrstring = LZ4F_getErrorName(header_size);