fix(replay): Streamline session creation/refresh

[mydea]

We've been using _loadAndCheckSession both in initial session setup as well as when checking for expiration of session.
This leads to some not-so-optimized stuff, as we kind of have to do double duty in there (e.g. we constantly re-assign the session etc).

This streamlines this by splitting this into:

• _initializeSessionForSampling(): Only called in initializeSampling()
• _checkSession(): Called everywhere else, assumes we have a session setup yet

Only the former actually looks into sessionStorage, the latter can assume we always have a session already.

This also extends the behavior so that if we fetch a buffer session from storage and segment_id > 0, we start the session in session mode. Without this, we could theoretically run into endless sessions if the user keeps refreshing and keeps having errors, leading to continuous switchovers from buffer>session mode.

[github-actions]

size-limit report 📦

Path	Size
@sentry/browser (incl. Tracing, Replay) - Webpack (gzipped)	75.19 KB (+0.46% 🔺)
@sentry/browser (incl. Tracing) - Webpack (gzipped)	31.17 KB (+0.2% 🔺)
@sentry/browser - Webpack (gzipped)	21.85 KB (+0.19% 🔺)
@sentry/browser (incl. Tracing, Replay) - ES6 CDN Bundle (gzipped)	69.72 KB (+0.27% 🔺)
@sentry/browser (incl. Tracing) - ES6 CDN Bundle (gzipped)	28.18 KB (+0.1% 🔺)
@sentry/browser - ES6 CDN Bundle (gzipped)	20.18 KB (+0.05% 🔺)
@sentry/browser (incl. Tracing, Replay) - ES6 CDN Bundle (minified & uncompressed)	219.95 KB (+0.41% 🔺)
@sentry/browser (incl. Tracing) - ES6 CDN Bundle (minified & uncompressed)	84.78 KB (+0.13% 🔺)
@sentry/browser - ES6 CDN Bundle (minified & uncompressed)	59.86 KB (+0.04% 🔺)
@sentry/browser (incl. Tracing) - ES5 CDN Bundle (gzipped)	31.04 KB (+0.09% 🔺)
@sentry/react (incl. Tracing, Replay) - Webpack (gzipped)	75.21 KB (+15.62% 🔺)
@sentry/react - Webpack (gzipped)	21.88 KB (+0.18% 🔺)
@sentry/nextjs Client (incl. Tracing, Replay) - Webpack (gzipped)	93.03 KB (+0.41% 🔺)
@sentry/nextjs Client - Webpack (gzipped)	50.7 KB (+0.19% 🔺)

[billyvg]

LGTM, will do some manual testing tomorrow

[billyvg]

Hmm do we not use codecov in this repo?

[billyvg]

Seeing an issue where we have stale events being flushed alongside the new session, Francesco will be looking into this tomorrow.

[mydea]

Note: I updated this PR to fix another bug:

Now, when we restore a session for sessionStorage, we only start in buffer mode if sampled === buffer and segmentId === 0. This means that if we have a buffer that was previously converted to a session mode due to an error, when the user reloads they will continue in session mode, not in buffer mode. Without this, you could theoretically have endless sessions if you continuously have errors, and reload, without ever reaching the session max. time before reloading.