Parse unix timestamps based on MSB

[guilload]

Position of the MSB:

• 28-33 -> seconds
• 38-43 -> millis
• 48-53 -> micros
• 58-63 -> nanos

TODO

• [ ] implement MSB-based UNIX timestamp parsing
• [ ] add unix_timestamp back to default parsers
• [ ] clean up code (remove references to unix_ts_secs, unix_ts_millis, ...)
• [ ] update docs