Upgrade min versions of tokio, tracing-subscriber, and criterion (#1662)

* Upgrade minimum Tokio version to 1.8.4

This addresses RUSTSEC-2021-0072 and RUSTSEC-2021-0124.
The SDK, runtime crates, and server are set to the minimum secure
version of Tokio to allow a larger range of versions to be used by
consumers of those libraries. The tools and the Python server are
both set to the latest Tokio version since they are not intended
to be consumed as a libraries.

* Upgrade `tracing-subscriber` to 0.3.15

* Upgrade `criterion` to 0.3.6

This addresses RUSTSEC-2021-0093 by upgrading `criterion`, which
upgrades `rayon`, which upgrades `crossbeam-deque`.