This PR changes the way we aggregate and flush stats in the concentrator.

What it does.

• We no longer drop traces in agent.Process. The concentrator takes care of handling traces of any time itself.
• The previous constant of "2 active time buckets" is moved to bufferLen. You can see it previously used in if model.Now()-2*a.conf.BucketInterval.Nanoseconds() and if ts > now-2*c.bsize [1]. It still defaults to 2.
• bufferLen drives two things: it is the number of active time buckets for which we ingest statistics and the age of the time buckets we flush (aka. the delay before flushing a bucket).
• During a flush, we move from bufferLen +1 active buckets (2 in the past + the current one) to bufferLen (the current one + the old one). That's the same behavior as before.
• During a flush, we flush only the buckets before now-c.bufferLen*bsize, like before. It means a latency of 1 to 2 bsize aka. 10s to 20s.
• In practice, as a flush is triggered every bsize, we only flush one bucket, the one at alignedNow-c.bufferLen*bsize.
• To deal with old spans (aka. older than oldestTs, where now + bufferLen*c.bsize < oldestTs < now + (bufferLen-1)*c.bsize) we basically count the span as if its time was oldestTs. It means any old data ends up in the oldest bucket we kept.
• At boot, the Agent is stricter, only allowing/aligning stats to the current time bucket. That allows it not to flush a bucket for a time window it would have flushed before an Agent restart.

[1] bsize is the length of a time bucket, it's isn't publicly configurable so consider it as always 10 seconds (there is a TODO in the code to remove its configuration interface).

It practice, we can summarize it to two core changes:

• We no longer create time buckets for spans arbitrarily far in the past (but with a recent root). That was leading to arbitrarily late stats buckets, wrongly processed in the pipeline / overriding what got flushed in the past. Instead, its stats are now counted in the oldestTs bucket.
• We no longer drop stats for traces with a root late in the past. Consistently with the previous point, it is counted in the oldestTs bucket.

TODOS left:

• Configurable bufferLen.