write_http: adapt the behaviour of Timeout option

write_http: adapt the behaviour of Timeout option

- rename PostTimeoutSec parameter to Timeout

- change Timeout to take milliseconds rather than seconds, for the sake
  of consistency: collectd allow working at the sub-second scale if need
  be, so it's a shame to restrain this here.

- do *not* set the default timeout to the value of Interval. Rationale:
  what matters here is that each POST request completes before the send
  buffer fills up again. How fast the send buffer fills up completely
  depends on how much data is collected. Interval is unrelated to this,
  and unlike read plugins, I don't think it makes a good value to use as
  the default timeout.