Add ability to manually specify timestamp

[srgykuz]

Here f2618e0 you removed ability to manually specify Timestamp in favor of formula timestamp = receivedAt - (sentAt - originalTimestamp).

During SDK use I'm expecting that server will use timestamp that I specified. I don't want server to use different timestamp. Above change introduces these problems:

1. it is not always obvious, until you dive into documentation, what field to use for timestamp. RudderStack documentation mentions 3 fields: timestamp, originalTimestamp and sentAt. Expected behavior is simple: you passing timestamp, server using it. But actual behavior is that server using above formula to calculate timestamp, which results in different timestamp than actually was passed.
2. documentation allows to manually specify timestamp, while SDK don't allows it

My proposal is to return Timestamp field back and leave decision up to client to decide what field for timestamp to use.