SearchIdx: Add the mechanism for making all Xapian indexing optional
Create a new method add_xapian that holds all of the code to create
Xapian indexes. The creation of this method simpliy involved
idenitifying the relevant code and moving it from add_message.
A call is added to add_xapian from add_message to keep everything
working as it currently does. The new call is made conditional upon
index levels of 'full' and 'medium'. The index levels that index
positions and terms the two things public-inbox uses Xapian to index.
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>