ongoing...
Fix: float value must be correctly encoded.

str(float_value) would produce things such as '1.156464+e6' while we want '1156464000000'.
clean
Misc enh: use_decicated_thread optionnaly. Currently disabled.

+ purge perfdata after 3*interval if no more new data.
+ still report command result even if some perfdata weren't updated:
  If at least one perfdata was updated then continue reporting of perf data.