Fix types inconsistency on API level with time

[timocov]

Currently we have several "form" of time:

• inbound time - it might be "business day string", "business day object" and "utc timestamp"
• outbound time - even if we declared it as Time (what means the same as "inbound time"), but actually it might be "business day object" and "utc timestamp" only, because we strip strings on API level and convert them in "business day object"
• "time comes from model level" - that times, which are generated on model level, which knows nothing about Time and knows about "business day object" and "utc timestamp" (the same as outbound one, but more explicit on type level)

I think we need to make it more consistency and leave the only one possible time value (whatever it will be) (or maybe two of them, but split them out and make somehow).

In internal discussion @eugene-korobko suggests to use "inbound time" everywhere, and provide the same time value an user passed to the library (currently we parse/convert it and just forget about the original value). But there is question "what we need to do if we have 2 series, one has data with "business day object" and another one with "business day string", and they have the same time point, what value we should provide here? Which was set first?

Me personally thinks that it might be better to split explicitly "inbound" and "outbound" types and use them (actually it's pretty the same we currently have, but without having wrong types somewhere).