Consolidate chained qd_buffer_t field handling code

There are many places in the router's C codebase where we need to work with data spanning a chained qd_buffer_t list. In particular the low level operations such as:

• advance N bytes
• compare N bytes
• copy N bytes
  are common throughout the code.

Here are examples from the iterator.c library:
copy bytes
advance cursor
compare

Similar logic is implemented in message.c and parse.c.

We should de-duplicate this code by providing a common implementation as part of the buffer handling code.