Add strict from and until times verification in api

[grzkv]

Currently, api accepts some times that are either too liberal or just incorrect. In this case, the defaults are applied that either are not obvious or include timeframe that starts "at the beginning of time" (for Unix).

E.g.

http://localhost:8081/render?target=a.a.a&from=+5m (future)
http://localhost:8081/render?target=a.a.a&until=+5m (from implied to come since forever and into the future)

There are more similar cases.

This is closely related to #160

[grzkv]

@KozzyKoder You may be interested in this one.

[azhiltsov]

Also would be nice to prevent swap of from and until
from go-carbon logs:
Invalid time interval: from time '1562536741' is after until time '1561932061'

[bom-d-van]

the issue is resolved by #200 and #341 now.