Allow indexing cleartext of encrypted messages (v4)

[Daniel Kahn Gillmor]

This is the fourth draft of the series that enables indexing cleartext
of encrypted message parts.

previous versions start at:

v1:  id:1449718786-28000-1-git-send-email-dkg at fifthhorseman.net
v2:  id:1453258369-7366-1-git-send-email-dkg at fifthhorseman.net
v3:  id:1454272801-23623-1-git-send-email-dkg at fifthhorseman.net

differs from v3 in that it uses Bremner's "message properties" to
record its information instead of trampling on the user-visible tag
space.

It depends also on one additional fix i pushed to the "message
properties" series, allowing notmuch queries with a "has:" prefix to
search the property namespace.  In particular:

 id:1467969336-7605-1-git-send-email-dkg at fifthhorseman.net

I welcome feedback!