client: Create a new HTTP session for each request

[underyx]

The globally shared connection resets after some time. When it's reset, the web worker will optimistically try to use it anyway, and fail with a connection error.

WIth many web workers running behind a load balancer, this means that a huge portion of login requests would randomly land on a broken session and it was common for people to need to retry 3-4 times until their login attempt finally worked.

You can see here how within the sentry project's own code, sessions are created on demand instead of being shared globally: https://github.com/getsentry/sentry/blob/27cc0fed47732dec907668a4529ca39bb384bf5a/src/sentry/http.py#L271

[underyx]

@SkyLothar are you still active? Any chance to get this merged and released officially?