sql_find()
  Recipient email address to index scanner, operating on SQL database.
  
  This scanner is an reimplementation of e2i_sql lmtpd() service from
  older sagator 0.7.2. It is useful only as lmtpd() scanner, because
  it operates on email recipients.
  
  Usage: sql_find(key, dbc, query, scanners)
  
  Where: key is an string, which defines which variable to compare.
           Currently only "recipient" or "sender" can be used here.
         dbc is an database connection
         query is an SQL condition to use
         scanners is an dictionary of scanners, each returned key from
           database must have coresponding key in this dictionary
  
  Example: sql_find(
             'recipient',
             db.sqlite(),
             "SELECT key FROM userpref WHERE email=%s",
             {
               'AV': b2f(libclam()),
               'AS': spamassassind(),
               '': s2f(libclam())+spamassassind() # default
             }
           )
  
  Limitations: It is not possible to include one sql_find() into another.
  
  New in version 0.9.0.