SDS: use type 8 if we are likely to append to the string.
SDS: use type 8 if we are likely to append to the string.

When empty strings are created, or when sdsMakeRoomFor() is called, we
are likely into an appending pattern. Use at least type 8 SDS strings
since TYPE 5 does not remember the free allocation size and requires to
call sdsMakeRoomFor() at every new piece appended.

JackieXie168/redis,JackieXie168/redis,JackieXie168/redis,JackieXie168/redis