Optimistic session creation

[juancampa]

Problem

Launch takes too long

Solution

Not a complete solution but an improvement.

Normally a window is created, and when it's ready it sends a new message. When the main process receives the new message, it creates a new session.

Instead of waiting for the new message, the main process creates a session optimistically so that when the new messages is actually received, a session is already warmed up.

Benchmarks

This measures the time from when app/index.js first runs until the first session data message is sent to the renderer. Ran it 5 consecutive times.

canary: 749, 728, 765, 777, 762
this branch: 617, 656, 615, 590, 596

Averages:

canary	this branch	diff
756ms	614ms	142ms

Not a hell of a lot but doesn't hurt either. I've only tested on Linux, but should help even more on other platforms

[rauchg]

Succinct and effective!

[juancampa]

I also did a video recording (thanks @chabou!) to verify that the actual "time to prompt" was improved and got results that are consistent with the measurements above: ~150ms speed-up on Linux