nmhive.py: Use notmuch behind GET /tags

nmhive.py: Use notmuch behind GET /tags

I stick with the NMBPREFIX environment variable for consistency with
nmbug itself.  I also drop 'app.debug = True', to avoid trouble like:

  $ ./nmhive.py
   * Running on http://0.0.0.0:5000/
   * Restarting with reloader
  A Xapian exception occurred opening database: Unable to get write
    lock on /.../xapian: already locked
  Traceback (most recent call last):
    File "./nmhive.py", line 83, in <module>
      mode=notmuch.Database.MODE.READ_WRITE)
    File "/.../notmuch/database.py", line 154, in __init__
      self.open(path, mode)
    File "/.../notmuch/database.py", line 214, in open
      raise NotmuchError(status)
  notmuch.errors.XapianError

because with 'debug = True', Flask tries to run two instances of this
process simultaneously, but only one can hold the write lock at a
time.  If we want to scale this up to multiple writing
threads/processes, we'll probably want to make the persistant Database
instance read-only, and either acquire a write lock as necessary, or
just instantiate a read/write database for each PUT.  For now, it's
easy enough to just have a single thread.