Fix filesystem layout timestamps with milliseconds

[sultaniman]

This PR uses datetime.timestamp() for timestamps with milliseconds instead of pendulum.format().
I thought it was fixed but apparently got mixed up.

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[rudolfix]

Please tell me what was the bug really? "SSS" is a milisecond part of a datetime, not a full timestamp? hard to review without knowing what got fixed
otherwise LGTM

[sultaniman]

@rudolfix yes it SSS is just a millisecond part of it and pendulum has x but it is a stringified timestamp with seconds and now it is just the old way of doing timestamp*1000. Switching between the tasks I overlooked this. Sorry.

[sh-rp]

looks good, maybe one additional request: put an example in the docs. Right now it says "timestamp_ms - the current timestamp in Unix Timestamp format rounded to milliseconds" which is not quite right, because it is not a unix timestamp rounded to ms (this would look like this: 12334455.233) but it looks like this: 12334455233 actually.

PS: Maybe it is considered a unix timestamp, that's my bad, but an example would be good.