SearchIdx: Allow the amount of indexing be configured
This adds a new inbox configuration option 'indexlevel' that can take
the values 'full', 'medium', and 'basic'.
When set to 'full' everything is indexed including the positions
of all terms.
When set to 'medium' everything except the positions of terms is
indexed.
When set to 'basic' terms and positions are not indexed. Just the
Overview database for NNTP is created. Which is still quite good and
allows searching for messages by Message-ID. But there are no indexes to support
searching inside the email messages themselves.
Update the reindex tests to exercise the full medium and basic code paths
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>