The TryParseTimestampRFC3339Nano() must properly parse RFC3339 timestamps with timezone offsets.
While at it, make tryParseTimestampISO8601 function private in order to prevent
from improper usage of this function from outside the lib/logstorage package.
Updates https://github.com/VictoriaMetrics/VictoriaMetrics/issues/6508
Change the return values for these functions - now they return the unmarshaled result plus
the size of the unmarshaled result in bytes, so the caller could re-slice the src for further unmarshaling.
This improves performance of these functions in hot loops of VictoriaLogs a bit.
