fix: Clear activeTransaction from the scope and always start idle timers

[kamilogorek]

This makes sure, that after sampling a transaction, all future transactions are still captured correctly.

[github-actions]

size-limit report

Path	Size
@sentry/browser - CDN Bundle (gzipped)	20.33 KB (-0.01% 🔽)
@sentry/browser - Webpack	21.21 KB (0%)
@sentry/react - Webpack	21.23 KB (0%)
@sentry/browser + @sentry/tracing - CDN Bundle (gzipped)	27.26 KB (+0.01% 🔺)

[lobsterkatie]

Can we add a it("finishes unsampled transactions") test and a it("removes unsampled transactions from the scope when finished") test, since those are what this PR aims to fix?

Otherwise LGTM. 🙂

[kamilogorek]

Can we add a it("finishes unsampled transactions") test

I think it's covered by finishes a transaction after 3 beats test.

and a it("removes unsampled transactions from the scope when finished") test

Added.