Add export timestamps; distinguish Accumulation vs. Record

[jmacd]

This PR is factored out of a successor to #799.

In the export API:

• Existing Record type becomes Accumulation, having the same fields.
• Metadata struct is factored out, common part of both Record and Accumulation
• New Accumulation type is Metadata + Aggregator, used for Accumulator->Integrator
• New Record type is Metadata + Aggregation + timestamps, used for Integrator->Exporter

In the simple Integrator:

• Add a StartCollection() method
• Add an error to FinishCollection, used for checking the call sequence (and in the future to support errors from Merging aggregators)

In the OTLP code path:

• Add timestamps to the tests. Currently the assumption is that all timestamps are delta oriented. In the follow-on change, SumObserver and UpDwnSumObserver will change their behavior.

[jmacd]

For the record, the replacement branch for #799 contains the changes here plus the rest of #799:
https://github.com/jmacd/opentelemetry-go/tree/jmacd/799v2

[MrAlias]

This is a partial review, but I wanted to get some comments in now.
I think the overall strategy, but the Aggregator interface embedding the Aggregation interface seems troublesome. Otherwise, it looks good save for some variable renames (which I did not thoroughly exhaust).

[jmacd]

@MrAlias before you look again, I will check over the variable names of Accumulation variables that might be named like Records.

[jmacd]

(I've audited all uses of "Accumulation" for variable name and comment correctness.)

[MrAlias]

Looks good 👍