Additional config.c refactory and bugfixes

- add capability for each config to have a callback to check if value is valid and return error string
  will enable converting many of the remaining custom configs into generic ones (reducing the x4 repetition for set,get,config,rewrite)
- add capability for each config to  to run some update code after config is changed (only for CONFIG SET)
  will also enable converting many of the remaining custom configs into generic ones
- add capability to move default values from server.h and server.c to config.c
  will reduce many excess lines in server.h and server.c (plus, no need to rebuild the entire code base when a default change 8-))

other behavior changes:
- fix bug in bool config get (always returning 'yes')
- fix a bug in modifying jemalloc-bg-thread at runtime (didn't call set_jemalloc_bg_thread, due to bad merge conflict resolution (my fault))
- side effect when a failed attempt to enable activedefrag at runtime, we now respond with -ERR and not with -DISABLED

src/server.c

@@ -2295,7 +2295,6 @@ void initServerConfig(void) {
     server.active_expire_enabled = 1;
     server.active_expire_effort = CONFIG_DEFAULT_ACTIVE_EXPIRE_EFFORT;
     server.jemalloc_bg_thread = 1;
-    server.active_defrag_enabled = CONFIG_DEFAULT_ACTIVE_DEFRAG;
     server.active_defrag_ignore_bytes = CONFIG_DEFAULT_DEFRAG_IGNORE_BYTES;
     server.active_defrag_threshold_lower = CONFIG_DEFAULT_DEFRAG_THRESHOLD_LOWER;
     server.active_defrag_threshold_upper = CONFIG_DEFAULT_DEFRAG_THRESHOLD_UPPER;
@@ -2485,6 +2484,8 @@ void initServerConfig(void) {
      * script to the slave / AOF. This is the new way starting from
      * Redis 5. However it is possible to revert it via redis.conf. */
     server.lua_always_replicate_commands = 1;
+
+    initConfigValues();
 }
 
 extern char **environ;

src/server.h

@@ -171,7 +171,6 @@ typedef long long ustime_t; /* microsecond time type. */
 #define CONFIG_DEFAULT_LAZYFREE_LAZY_EXPIRE 0
 #define CONFIG_DEFAULT_LAZYFREE_LAZY_SERVER_DEL 0
 #define CONFIG_DEFAULT_ALWAYS_SHOW_LOGO 0
-#define CONFIG_DEFAULT_ACTIVE_DEFRAG 0
 #define CONFIG_DEFAULT_DEFRAG_THRESHOLD_LOWER 10 /* don't defrag when fragmentation is below 10% */
 #define CONFIG_DEFAULT_DEFRAG_THRESHOLD_UPPER 100 /* maximum defrag force at 100% fragmentation */
 #define CONFIG_DEFAULT_DEFRAG_IGNORE_BYTES (100<<20) /* don't defrag if frag overhead is below 100mb */
@@ -2077,6 +2076,7 @@ void resetServerSaveParams(void);
 struct rewriteConfigState; /* Forward declaration to export API. */
 void rewriteConfigRewriteLine(struct rewriteConfigState *state, const char *option, sds line, int force);
 int rewriteConfig(char *path);
+void initConfigValues();
 
 /* db.c -- Keyspace access API */
 int removeExpire(redisDb *db, robj *key);