Add the Asynchronous and Synchronous state packages for review LS-29754

[jmacd]

Description:
The internal/syncstate and internal/asyncstate package contain the adapters used by the MeterProvider to the viewstate package's compiler. The syncstate package, in particular, handles the hot code path used by synchronous instruments.

Link to tracking Issue:
LS-29760 describes the overall goal; this is one part.

Testing:
End-to-end testing was done. The alt_metrics_sdk/development branch has a assembled copy of this code. Depends on various Lightstep-side PRs merging to see the exponential histograms work.

[jmacd]

Reviewers: as discussed last week, there was still a TODO about a race condition; the synchronous-instrument delta-temporality collector was not correctly synchronized for deletion to occur from its map. This could have been repaired in at least two ways; I chose the one that IMO is better, but this is subtle. This change is shown in the following commit:

54ac1b8

This needs more documentation and can use further testing, however it resolves the races and TODO. The design is as follows:

1. No change in the synchronous instrument hot path
2. Concurrent collection from synchronous instruments happens concurrently, up and through the accumulators
3. Allocators are (continue to be) added to the collector's map under a lock and released after SnapshotAndProcess w/o the lock
4. SnapshotAndProcess() takes a new boolean argument, saying if it is the final snapshot. This lets the collectors take advantage of the syncstate reference counting mechanism.
5. Collection takes the same lock (blocking new accumulators) and synchronously moves the data into the output
6. Then, as a special case, after the synchronous move, the zero value is skipped
7. If there was a zero value and the refcount is zero (since the lock is held), safe to remove the entry from the map.

[jaronoff97]

A few comments, but nothing blocking. 💯