Prevent fetching the user agent concurrently

[richardm-stripe]

r? @dcr-stripe
cc @stripe/api-libraries

Motivation

Fixes #1202

Summary

Right now, when calculating the proper user agent, we check a cache and shell out to uname if the cache is empty. This is bad because we can shell out to uname multiple times concurrently before the cache is populated. For some bulk operation usage patterns this causes an unwanted CPU spike (see #1202). It would be better to only shell out once.

This PR makes two changes:

1. Makes the cache use a promise. Promises are very good at resolving a value only once.
2. Caches the result of uname directly, instead of the derived user_agent. This allows me to remove some questionable cache invalidation. We should only ever need to call uname once, uname is not expected to change.

Breakingness:

Stripe.USER_AGENT_SERIALIZED isn't marked as private, but I think we should consider it safe to just remove this property. It was an internal cache and I cannot imagine any legitimate use case relying on it.

Tests

Existing tests for getClientUserAgent pass.

[dcr-stripe]

LGTM!