Desktop: Speed up WebDAV/Nextcloud sync by ~30% (or even more)

[roman-r-m]

(Sorry for the catchy title, couldn't help myself.)

Due to a typo Keep-Alive wasn't sent so each time http connection has to be re-established from scratch).
I have tested this with a server in my local network using plain http and got these results:

• Before the change

21:10:38: CommandService::execute: synchronize false
...
21:13:34: Sync: finished: Synchronisation finished [1614978638697]

almost exactly 3 minutes

• After the change

21:21:15: CommandService::execute: synchronize false
...
21:23:19: Sync: finished: Synchronisation finished [1614979275473]

~2 minutes, so 30% speedup. For https I expect the effect would be even greater. Not too bad for a one-symbol change.

Total synced volume was 188,621,312 bytes, items:

main-html.js:48 21:23:19: Operations completed: 
main-html.js:48 21:23:19: createRemote: 1264
main-html.js:48 21:23:19: fetchingTotal: 1265
main-html.js:48 21:23:19: fetchingProcessed: 1265
main-html.js:48 21:23:19: Total folders: 27
main-html.js:48 21:23:19: Total notes: 850
main-html.js:48 21:23:19: Total resources: 187

PS
Why is Linux-only? And why is httpAgent only used for WebDAV?

[roman-r-m]

Ran another test with Nextcloud over https:

• Before the change: ~1h 15 min to sync the same set of notes as above
• After the change: 35 min

[laurent22]

See here about why it's only enabled on Linux: #1931

[roman-r-m]

It actually does not explain why it's Linux only, the issue that the author of that PR was fixing (#1023) references Linux, presumably because the user who raised it was using Linux.

I will test it on my Windows machine (too bad I've deleted my dev environment there), but I do not see a reason why it should not work there.

[laurent22]

There was also a concern about whether HTTP agents work with React Native or not.

[roman-r-m]

There was also a concern about whether HTTP agents work with React Native or not.

That I have not tested. This PR just fixes a typo that made this original change pointless, even on Linux.

I still think it should be useful on Windows too, just having trouble building the desktop app at the moment.

[laurent22]

Ok I guess we can merge anyway now, just for the typo fix. It's true it might work just fine on other platforms too, but we'd need to test.

[roman-r-m]

It looks like Windows is also affected by the same issue. As you can see from the screenshot Joplin closes the connection after each request, all those SYN/ACK/FIN are just time wasted . I'll see if the same fix applies on Windows.