dict.c: added optional callback to dictEmpty().

Redis hash table implementation has many non-blocking features like
incremental rehashing, however while deleting a large hash table there
was no way to have a callback called to do some incremental work.

This commit adds this support, as an optiona callback argument to
dictEmpty() that is currently called at a fixed interval (one time every
65k deletions).

src/redis.h

@@ -1165,7 +1165,7 @@ void setKey(redisDb *db, robj *key, robj *val);
 int dbExists(redisDb *db, robj *key);
 robj *dbRandomKey(redisDb *db);
 int dbDelete(redisDb *db, robj *key);
-long long emptyDb();
+long long emptyDb(void(callback)(void*));
 int selectDb(redisClient *c, int id);
 void signalModifiedKey(redisDb *db, robj *key);
 void signalFlushedDb(int dbid);