FileSystemWatcher.Linux: use a single inotify instance and refactor watch tracking.

[tmds]

(By default) Linux allows 128 inotify instances per user. By sharing the inotify instance between the FileSystemWatchers we reduce contention with other applications.

Fixes #62869.

[tmds]

@dotnet/area-system-io @stephentoub ptal.

[adamsitnik]

@tmds big thanks for your contribution! For now I've reviewed 25% of this PR (I need to wrap my head around all the locks and it's going to take me a while), PTAL at my comments.

[tmds]

For the locking, this overview may be helpful:

• watcherslock: guards the list of Watcher
• lock on Watcher: guards its tree of WatchedDirectory
• lock on Watch: guards its list of WatchedDirectory
• addLock: this reader writer lock enables concurrent adding of watches ("readers"), but no watches may be added while an inotify watch is removed ("writer")

To prevent deadlocks, the locks (as needed) are taken in this order: watchersLock, addLock, lock on Watcher, lock on Watch.

[tmds]

@adamsitnik this is a challenge to review so feel free to ask any questions you have while looking at the code.

I'm also going to make some time this week to look at the PR with my reviewer's hat on.