Change timestamp accuracy from RFC3999Nano to RFC3339Micro

According to RFC5424 maximum digits allowed in TIME-SECFRAC is **6**.
Example 5 on http://tools.ietf.org/html/rfc5424#section-6.2.3.1 also
points out, that using nanosecond accuracy produces invalid timestamps

This causes problems when syslog-redirector is combined with external
tools (i.e. rsyslog). Nanoseconds are parsed incorrectly, producing
garbage output down the line.