fix(gatsby-plugin-gatsby-cloud): add backpressure for IPC

[vladar]

Description

Adds backpressure to IPC to prevent message loss in case of process exiting early. As mentioned in docs on process.send:

subprocess.send() will return false if the channel has closed or when the backlog of unsent messages exceeds a threshold that makes it unwise to send more. Otherwise, the method returns true. The callback function can be used to implement flow control.

Also related: nodejs/node#7657 (comment)

This PR also adds batching for emitRoute IPC calls.

P.S. GitHub diff here is pretty messy, use Files Changed -> Settings -> Hide whitespace characters to make it readable.

Caveat: Jest

Apparently, Jest sets up an incorrect stub for process.send which returns undefined: https://github.com/facebook/jest/blob/98f10e698ae986c19fef2d8117be2341bcfb8f7f/packages/jest-util/src/createProcessObject.ts#L113

Node always returns boolean from process.send (1, 2). To account for this - had to trick Jest by changing the original implementation to this.

Edit: submitted a PR to Jest with a proposed fix: jestjs/jest#11799