Introduction of a new string encoding: EMBSTR

Previously two string encodings were used for string objects:

1) REDIS_ENCODING_RAW: a string object with obj->ptr pointing to an sds
stirng.

2) REDIS_ENCODING_INT: a string object where the obj->ptr void pointer
is casted to a long.

This commit introduces a experimental new encoding called
REDIS_ENCODING_EMBSTR that implements an object represented by an sds
string that is not modifiable but allocated in the same memory chunk as
the robj structure itself.

The chunk looks like the following:

+--------------+-----------+------------+--------+----+
| robj data... | robj->ptr | sds header | string | \0 |
+--------------+-----+-----+------------+--------+----+
                     |                       ^
                     +-----------------------+

The robj->ptr points to the contiguous sds string data, so the object
can be manipulated with the same functions used to manipulate plan
string objects, however we need just on malloc and one free in order to
allocate or release this kind of objects. Moreover it has better cache
locality.

This new allocation strategy should benefit both the memory usage and
the performances. A performance gain between 60 and 70% was observed
during micro-benchmarks, however there is more work to do to evaluate
the performance impact and the memory usage behavior.

src/redis.h

@@ -174,6 +174,7 @@
 #define REDIS_ENCODING_ZIPLIST 5 /* Encoded as ziplist */
 #define REDIS_ENCODING_INTSET 6  /* Encoded as intset */
 #define REDIS_ENCODING_SKIPLIST 7  /* Encoded as skiplist */
+#define REDIS_ENCODING_EMBSTR 8  /* Embedded sds string encoding */
 
 /* Defines related to the dump file format. To store 32 bits lengths for short
  * keys requires a lot of space, so we check the most significant 2 bits of
@@ -1138,6 +1139,8 @@ void freeZsetObject(robj *o);
 void freeHashObject(robj *o);
 robj *createObject(int type, void *ptr);
 robj *createStringObject(char *ptr, size_t len);
+robj *createRawStringObject(char *ptr, size_t len);
+robj *createEmbeddedStringObject(char *ptr, size_t len);
 robj *dupStringObject(robj *o);
 int isObjectRepresentableAsLongLong(robj *o, long long *llongval);
 robj *tryObjectEncoding(robj *o);
@@ -1164,6 +1167,7 @@ int compareStringObjects(robj *a, robj *b);
 int collateStringObjects(robj *a, robj *b);
 int equalStringObjects(robj *a, robj *b);
 unsigned long estimateObjectIdleTime(robj *o);
+#define sdsEncodedObject(objptr) (objptr->encoding == REDIS_ENCODING_RAW || objptr->encoding == REDIS_ENCODING_EMBSTR)
 
 /* Synchronous I/O with timeout */
 ssize_t syncWrite(int fd, char *ptr, ssize_t size, long long timeout);