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.
Fix replicationFeedSlaves() to use sdsEncodedObject() macro.
Fixed a possible bug in client->reply_bytes computation.
Fix setDeferredMultiBulkLength() c->reply_bytes handling with EMBSTR

This function missed proper handling of reply_bytes when gluing to the
previous object was used. The issue was introduced with the EMBSTR new
string object encoding.

This fixes issue #1208.
Test: regression test for issue #1208.
getStringObjectSdsUsedMemory() function added.

Now that EMBSTR encoding exists we calculate the amount of memory used
by the SDS part of a Redis String object in two different ways:

1) For raw string object, the size of the allocation is considered.
2) For embstr objects, the length of the string itself is used.

The new function takes care of this logic.
Every function inside sds.c is now commented.
Inline protocol improved to accept quoted strings.
sdsrange() does not need to return a value.

Actaully the string is modified in-place and a reallocation is never
needed, so there is no need to return the new sds string pointer as
return value of the function, that is now just "void".
Ignore sdsrange return value.
Use latest sds.c in the hiredis library under deps.