Some changes related to UNITS

[kikoalbiol]

Make explicit the time units.

struct field_caller : public ts_caller {
            detail::field_caller& field(const std::string& k, const std::string& v)   { return _f_s(',', k, v); }
            detail::field_caller& field(const std::string& k, bool v)                 { return _f_b(',', k, v); }
            detail::field_caller& field(const std::string& k, short v)                { return _f_i(',', k, v); }
            detail::field_caller& field(const std::string& k, int v)                  { return _f_i(',', k, v); }
            detail::field_caller& field(const std::string& k, long v)                 { return _f_i(',', k, v); }
            detail::field_caller& field(const std::string& k, long long v)            { return _f_i(',', k, v); }
            detail::field_caller& field(const std::string& k, double v, int prec = 2) { return _f_f(',', k, v, prec); }
            detail::ts_caller& timestamp_ns(unsigned long long ts)                  { return _ts(ts); }
            detail::ts_caller& timestamp_us(unsigned long long ts)                  { return timestamp_ns(1000*ts); }
            detail::ts_caller& timestamp_ms(unsigned long long ts)                  { return timestamp_us(1000*ts); }
            detail::ts_caller& timestamp_s(unsigned long long ts)                  { return timestamp_ms(1000*ts); }
        };

[orca-zhang]

Thank you for your contributions. I'll check it and merge them then. : )