* Redo API client locking

This assigns local values when in critical paths, allowing a single API
client to much more quickly and safely pipeline requests.

Additionally, in order to take that paradigm all the way it changes how
timeouts are set. It now uses a context value set on the request instead
of configuring the timeout in the http client per request, which was
also potentially quite racy.

Trivially tested with
VAULT_CLIENT_TIMEOUT=2 vault write pki/root/generate/internal key_type=rsa key_bits=8192