ref(core): Prevent redundant setup work

[lobsterkatie]

This PR fixes two places where we do work unnecessarily:

1. When creating a new Hub instance we call bindClient, even if there's no client to bind.

2. When calling Client.setupIntegrations() (as we do when binding a client to a hub), we run through the whole integration-setup process, even if the client has already had its integrations set up.

(The latter situation comes up, among other places, any time we create a domain: each new domain gets its own hub, which then is bound to the current client, meaning we currently call setupIntegrations() for every incoming request handled by either our Express integration or our nextjs SDK.)

To prevent this, we now (as of this change) check a) if there's a client to bind to the new hub, and b) if the client having its integrations set up has already had that done.

[github-actions]

size-limit report

Path	Size
@sentry/browser - CDN Bundle (gzipped)	21.55 KB (+0.1% 🔺)
@sentry/browser - Webpack	22.55 KB (+0.1% 🔺)
@sentry/react - Webpack	22.58 KB (+0.1% 🔺)
@sentry/browser + @sentry/tracing - CDN Bundle (gzipped)	28.97 KB (+0.06% 🔺)