Update Envoy to dee7021 (Aug 18th 2021).

[mum4k]

• Envoy now demands that Envoy::Grpc::AsyncClient is created, used and destroyed on the same thread. (See
  [grpc]: fix ex_authz grpc client race condition envoy#17619). Nighthawk creates the Envoy::Grpc::AsyncClient on the worker thread, but destroys it on the main which triggers an assertion failure. Fixing this by:
  • Adding a new method RequestSource::destroyOnThread() which complements the pre-existing RequestSource::initOnThread(). While the latter gets called when a worker thread starts, the former now gets called when a worker thread is shutting down.
  • Implementing RequestSource::destroyOnThread() on the only request source implementation that uses Envoy::Grpc::AsyncClient, i.e. Nighthawk::RemoteRequestSourceImpl, the method destroys the object that
    owns Envoy::Grpc::AsyncClient when called.
• no changes to .bazelrc, .bazelversion, run_envoy_docker.sh.
• lowering the coverage threshold to 93.2 to accommodate the fact that this code path is only covered in integration tests. The unit-tests use a mock.

[oschaaf]

the coverage test seems to indicate we started trending low, I'd be surprised if this PR pushed it over the edge, could that be a flake somehow?
That aside, this LGTM

[mum4k]

Adjusted the coverage threshold and included explanation in the PR description.

PTAL