input/collectd: Add old-style interval type

  * Only added to the typemap, as get_values already handles the field
input/collectd: Remove unused function

  * The vt_map function was called for every field in a VALUES part,
    but the result was never used in the program.
input/collectd: Simplify packet parse loop logic

  * Generate the event if the current part is and interval or values
    part
  * Log a warning if the type-number of the part is unknown
input/collectd: Add extra event clean up code