-addddt
template: pers?clnt=<name|pass>&com=addddt&sid=<id>&<usr>&<ftr_1>=<timestamp_pattern_1>&<ftr_2>=...
          Order of query params may be important: among features whose
          timestamp pattern delegates the timestamping to PServer, the
          leftmost features are considered first when assigning a
          timestamp (as if being visited first by the user).
pattern : - | <date/time>, where '-' means that the PServer assigns a
          transaction timestamp, otherwise the <date/time> provided by
          the application is used. The <date/time> format must be a
          long integer giving the milliseconds passed since January
          1st, 1970 00:00:00.000 GMT.
descript: for the specified user and for each 'ftr' parameter in the
          request, a new tuple (user, feature, timestamp, sid) is inserted
          in table 'decay_data'. In case the timestamp for a feature
          is provided by the application, it denotes when the (user,
          feature) interaction took place. In case the timestamping is
          is delegated to PServer, it is the date/time of the
          insertion of the tuple in the DB (the transaction time).
          In both cases the timestamps can be used to put feature
          interactions in the order they occured. For this however,
          session timestamping of all features in a decay feature group must
          be consistent and correct (either PServer or application
          generated, not both / mixed). If a request contains many
          features and some of them are assigned to be timestamped
          by PServer, some may end up with identical timestamps.
          This does not create any problem as long as the features
          are different. The DB however cannot record more than one
          identical (user, feature, timestamp) tuples. In case some
          timestamps are the same, the order of the corresponding
          interactions will be arbitrary. If a timestamp pattern is
          not legal, or if the request leads to two or more identical
          tuples, or if a feature 'ftr' does not already exist in the
          'up_features' table (feature column of two tables is linked
          through a referential integrity constraint), 401 is returned.
          If the sid is ommited then the last sid is assumed to be the desirable          
          If the error code 401 is returned then no changes have taken
          place in the DB.
example : pers?clnt=name|pass&com=addddt&usr=kostas&advert.link16=-&page5.combo8.choice2=1005854668670
returns : 200 OK, 401 (fail, request error), 501 (fail, server error)
200 OK  : in this case the response body is as follows
          <?xml version="1.0"?>
          <?xml-stylesheet type="text/xsl" href="/resp_xsl/rows.xsl"?>
          <result>
          <row><num_of_rows>number of relevant rows</num_of_rows></row>
          </result>
comments: the reference to the xsl file allows to view results
          in a web browser. In case the response body is handled
          directly by an application and not by a browser, this
          reference to xsl can be ignored.
          Note that the contents of the table 'decay_data' are not
          constraint or in any other way affected by the table
          'decay_groups'.