Make single instance the default

[timfish]

Running multiple instances of Electron is generally a bad idea and not really supported with the default configuration. Some renderer features don't work in this scenario (localStorage, IndexedDb, persisted sessions?) and this is not documented and so I'm guessing not tested for regressions either.

This would be a significant breaking change but in the spirit of guiding developers in the right direction it might be worth it.

Related to @MarshallOfSound's New event for makeSingleInstance.

[robatwilliams]

There also seems to be an issue with caching - I have reports that once an instance is running, subsequent launched instances fetch all network resources from the server rather than the cache.

I think the default should be to disallow doing unsupported things, especially if those are easily/accidentally done and on the face of it appear to work but with underlying issues.