stats: use internal context to support StatsClient stats

Moving to an internal context would allow us to avoid contention and reduce memory usage whenever we record stats. One such example may be found here:
https://github.com/envoyproxy/envoy/blob/master/source/common/http/user_agent.h

Note that the context itself would need to be guarded by a mutex, but stats artifacts it produced could theoretically then be used safely from any thread, avoiding needless dispatch.

Thank you to @jmarantz for graciously sharing his thoughts and insight on this topic.