Implement cancellation of NGINX Plus API calls

[pleshakov]

#1469 brought support for dynamic reconfiguration of NGINX Plus upstreams.
However, when making NGINX Plus API call (HTTP requests), we don't control they cancelation of those operations.

We'd like to control cancelations for two reasons:

• graceful termination. When NGF terminates, it needs to finish any in-progress API calls.
• configuring a API call timeout -- control how much time NGF allows for a successful API call

Acceptance criteria:

• Implement cancelation.
• Implement a reasonable timeout.

Blocked by nginx/nginx-plus-go-client#205

[sjberman]

Looks like nginx/nginx-plus-go-client#383 added support for this in the library, so we just need to pull those changes into our code.

[sjberman]

No longer relevant with the 2.0 architecture.