Timestamps in the playlist requests should be aligned

[mspanc]

Now typical request to the playlist looks like

https://plumber.radiokitapp.org/api/rest/v1.0/media/input/file/radiokit/vault?a[]=id&a[]=name&a[]=file&a[]=cue_in_at&a[]=cue_out_at&a[]=cue_offset&a[]=fade_in_at&a[]=fade_out_at&s[]=cue%202017-01-23T18%3A33%3A32.299Z%2020%20600&c[references][]=deq%20broadcast_channel_id%206f394f81-ee6b-4e8b-bfd9-d6ef968f1e3e&o[]=cue_in_at%20asc

Due to precision of cue in/cue out timestamps, there's no easy way to perform caching of such requests.

They should be aligned to nearest 5 or 15s block.