If the response doesn't have body tempesta parses the body until the connection
is closed, in this case need to update overall body size and don't make
chunks.

-Added error handling to `tfw_cache_h2_copy_body()`

There is copule of places where need to iterate over http message body.
For this purposes has been added body iterator. Just initialize
iterator and chunk using `tfw_body_iter_init()` with error handling.
Use `TFW_BODY_ITER_WALK` macro for walking over the body as TfwStr.

tfw_apm_hm_srv_alive(): Calculate crc32 for message body using body
itertor instead of looping invalid TfwStr.