MIME Field Free List

I've been reviewing some performance issues in the MIME support and I think there is a tweak that would improve performance and able to revert the change in #5293. Basically the MIME header would maintain a free list of fields in existing `MIMEHdrFieldBlock` instances. This should be easy as the `m_next_dup` pointer could be used as the free list next pointer. This is a pattern used elsewhere, most notably for the cache directory entries. This would minimize allocations and limit the length of the field block list