AOF: enable RDB-preamble rewriting by default.

There are too many advantages in doing this, RDB is faster to persist,
more compact, much faster to load back. The main issues here are that
the code is less tested because this was not the old default (so we are
enabling it for the new 5.0 release), and that the AOF is no longer a
trivially parsable format from now on. However the non-preamble mode
will be supported in the future as well, if new data types will be
added.

redis.conf

@@ -778,10 +778,7 @@ aof-load-truncated yes
 # When loading Redis recognizes that the AOF file starts with the "REDIS"
 # string and loads the prefixed RDB file, and continues loading the AOF
 # tail.
-#
-# This is currently turned off by default in order to avoid the surprise
-# of a format change, but will at some point be used as the default.
-aof-use-rdb-preamble no
+aof-use-rdb-preamble yes
 
 ################################ LUA SCRIPTING  ###############################
 

src/server.h

@@ -139,7 +139,7 @@ typedef long long mstime_t; /* millisecond time type. */
 #define CONFIG_DEFAULT_AOF_FILENAME "appendonly.aof"
 #define CONFIG_DEFAULT_AOF_NO_FSYNC_ON_REWRITE 0
 #define CONFIG_DEFAULT_AOF_LOAD_TRUNCATED 1
-#define CONFIG_DEFAULT_AOF_USE_RDB_PREAMBLE 0
+#define CONFIG_DEFAULT_AOF_USE_RDB_PREAMBLE 1
 #define CONFIG_DEFAULT_ACTIVE_REHASHING 1
 #define CONFIG_DEFAULT_AOF_REWRITE_INCREMENTAL_FSYNC 1
 #define CONFIG_DEFAULT_MIN_SLAVES_TO_WRITE 0