(plugin-server-session): delay initialization until first use

[hamudi-ahmed]

Goal

Serverless node plugins such as AWS Lambda (and the incoming Cloudflare Workers plugin) use plugin-browser-session and not plugin-server-session, but because plugin-server-session is bundled with the node notifier as an internal plugin, it's session tracking is still initialized on start, before any of the external plugins are loaded.

This causes problems in some environments like Cloudflare Workers, and results in unnecessary work and leaked timers, since the session delegate is replaced when those plugins are loaded.

Design

This PR fixes the issue by delaying the session tracker initialization until startSession is actually called - this preserves the existing behaviour for 'regular' node environments while allowing the serverless plugins to load plugin-browser-session before any initialization happens.

Testing

Covered by existing tests

[github-actions]

@bugsnag/browser bundle size diff

	Minified	Minfied + Gzipped
Before	47.74 kB	14.17 kB
After	47.74 kB	14.17 kB
±	No change	No change

code coverage diff

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[gingerbenw]

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