grpc-js: transition out of TRANSIENT_FAILURE if backoff timer has ended

[murgatroid99]

If the backoff timer ends before the connection attempt fails, the subchannel needs to transition into TRANSIENT_FAILURE briefly to signal to the channel that the connection attempt failed, but then it needs to transition out of TRANSIENT_FAILURE to be able to try to connect again.

This fixes #1434.

[nicolasnoble]

Hm. I think I would've liked it slightly better if there was a way to restart the backoffTimeout with a deadline of "now", which would've essentially streamlined this.

[murgatroid99]

Fundamentally, what's happening here is that the backoff timer already triggered, but the result of the timer was deferred until this state transition. It's important here that the backoffTimeout is not reset, and that it keeps any backoff it has accumulated.

[sidd-kulk]

package.json:

"@google-cloud/pubsub": "^1.1.6",
    "@grpc/grpc-js": "^1.1.7",
package-lock.json

"@google-cloud/pubsub": {
      "version": "1.7.3",
      "resolved": "https://registry.npmjs.org/@google-cloud/pubsub/-/pubsub-1.7.3.tgz",
...
"@grpc/grpc-js": {
          "version": "1.0.5",
          "resolved": "https://registry.npmjs.org/@grpc/grpc-js/-/grpc-js-1.0.5.tgz",
          "integrity": "sha512-Hm+xOiqAhcpT9RYM8lc15dbQD7aQurM7ZU8ulmulepiPlN7iwBXXwP3vSBUimoFoApRqz7pSIisXU8pZaCB4og==",
          "requires": {
            "semver": "^6.2.0"
          }
        }