(warning: bikeshed) Naming mergeinfo.db.

The current mergeinfo backend creates a SQLite database called
'mergeinfo.db'. However, as I understand it, it's not the canonical
source for mergeinfo (that's the properties on the noderevs).
mergeinfo.db is just an index, and could (and probably should, for other
reasons) be regenerated by e.g. svnadmin recover.

Secondly, I wonder what we'll do when we want to add more derived
information to the filesystem (e.g. someone [I forget who, sorry] showed
a proof-of-concept SQLite property indexer a while back). Will we
create another SQLite database?

So, I propose we rename mergeinfo.db to something more generic: say,
indexes.db. You can all lynch me now.

Regards,
Malcolm