Shmemfix2 review: support for faster shmem loading

I'd like to get the shmemfix2-review branch reviewed and hopefully integrated.  (I rebased shmemfix after pushing it to github, so using a clean, different branch should be better for review; the content is identical).

It enhances the share memory/mmap support so the loading/updating can be faster exploiting the underlying data source:

    if in-memory SOA seria is identical to that of the data source, load is skipped
    if these are different but the difference between these two versions can be retrieved from the data source, apply the diff instead of loading the entire zone

To implement the latter, this branch also includes the completion of zone data removing.

The branch roughly corresponds to the following BIND 10 tickets:
https://bind10.isc.org/ticket/2913
https://bind10.isc.org/ticket/2860
https://bind10.isc.org/ticket/2752
https://bind10.isc.org/ticket/2912

and the end of each of the corresponding part is marked in the commit log with the number. For example, [shmemfix2/2860] is the last commit for the part corresponding to ticket #2860. Review can be done incrementally for each part.