Context optimization for agent

[trask]

In profiling, I'm seeing a good amount of time spent reading the (current) span and the SERVER span from the context.

It may be possible to optimize this by using a context implementation in the agent which has dedicated fields for (current) span and SERVER span.

I think this would be worth it, since we expect these two fields to be present most of the time.

I'm not sure if this would be enough (with special checking in get and with on the passed in key), of if we would further want to make Span and ServerSpan use the optimized Context directly somehow (not through generic get and with).

[trask]

Closing, can revisit later if anyone has renewed interest.