-setdcy
template: pers?clnt=<name|pass>&com=setdcy&<ftr_group_1>=<decay_rate_1>&...
          Order of query params is important: position of 'com'
          is not important, however updates of feature group decay
          rates are performed in the order they appear in the request.
          Decay rate must exist and be a number from 0 to 1, both
          inclusive (eg: 0.5). Feature group is a template path
          expression 'pathname', representing all features with name
          starting with 'pathname'. Feature group cannot be "*".
descript: if the feature group already exists in the DB, the
          corresponding decay rate is updated to the new value. If the
          feature group is a new group, the new (group, rate) pair is
          inserted in the DB. If the error code 401 is returned, (eg. in
          case some decay rate was not numeric or was not in the proper
          range, or in case the feature group was illegal, eg. "*") then
          no changes at all have taken place in the DB. Note however
          that in case a feature group is the empty string, the
          corresponding (group, rate) pair will just be ignored.
example : pers?clnt=name|pass&com=setdcy&page10.banners=0.5&the_times.politics=1
          pers?clnt=name|pass&com=setdcy&frequency.allfeatures=0  (see comments)
returns : 200 OK, 401 (fail, request error), 501 (fail, server error)
200 OK  : no response body exists.
comments: - Note that if rate is set to 0, the decay calculated for
          a feature is reduced to the total number of visits the
          user paid to the feature.
          - Note that the same feature may be organized as part of
          two different hierarchies, eg. page10.banners.banner2,
          frequency.allfeatures.banner2, with possibly different
          rates. In this case, when the feature is selected by a
          user, the application must notify the server about the
          interaction for both (or more) names of the feature.
          - Note that it is possible to define overlapping sets of
          features, or sets who are subsets of other sets, giving
          possibly different rates for each, as in:
            pers?clnt=name|pass&com=setdcy&page10=0.3&page10.banners=0.6
          This will not result in more user-feature interactions
          recorded in the database (as is the case in the previous
          situation). The interactions are recorded strictly as the
          application dictates. However, this makes it posible for
          the application to query the same interaction data in
          different ways. By defining various overlapping and
          competing feature groups and assigning them different
          decay rates, the application may get different decay
          values for a (feature, user) pair.
          - Note that it is not necessary to register a feature
          group in 'decay_groups' in order to use the decay
          functionality. The decay service is based only on
          'decay_data' table, and populating this table has
          nothing to do with defining feature groups. It is
          possible to define s group and a corresponding decay
          rate on the fly, in the request that calculates the
          decay values. The table 'decay_groups' is just a
          convenient place to store groups and rates in order
          not to repeat them in every 'caldcy' request.