Minor refactor to conn_read_packet_body
Minor refactor to conn_read_packet_body

* Add more comments
* Only fetch the error code when it is an error
* Only log the reason why we failed to read from the socket
  before closing the connection. The pointers inside the
  buffers will be freed anyway (and reused) as part of
  closing the connection so they're of limited value.

Change-Id: I5e5222931ea297cb20c3e997efbc8db45190ab98
Reviewed-on: http://review.couchbase.org/82111
Tested-by: Build Bot <80754af91bfb6d1073585b046fe0a474ce868509@couchbase.com>
Reviewed-by: Oliver Downard <2b519a6943d9144e0afb0b62d71fdc5e98fa8017@couchbase.com>

daverigby/kv_engine,daverigby/kv_engine,daverigby/kv_engine,daverigby/kv_engine