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.