message properties patches, v1.0

[David Bremner]

There have been two rounds of RFC/WIP

      id:1463927339-5441-1-git-send-email-david at tethera.net
      id:1464608999-14774-1-git-send-email-david at tethera.net

This is the first feature complete version, and (compared to the
previous one) includes docs, more tests, and dump/restore support.

There is one notable API change: I added a convenience function to
remove all tags. More importantly I adjusted the memory semantics of
the iterators so that the underlying map objects are kept alive. This
is the same issue alread present in the tag iterator code; when the
message metadata cache is invalidated, we don't want the iterator to
start segfaulting, otherwise the iterators are effectively read-only.