🏗 Reduce gulp dist parallelism on Travis, add some logging

[rsimha]

Still trying to figure out why gulp dist occasionally stalls on Travis. Tried a few things listed in #24487.

Next steps in this PR:

• Reduce parallelism for multi-pass gulp dist to 1 (doesn't make a substantial difference to running time)
• Add logging + timing for buildExtensions() and compileAllJs() (so we know when all promises have resolved)
• Fix bug in timing each minification step (we were counting queued time instead of running time, and weren't including the minutes value for longer tasks)

See #24487 (comment)

Potential fix for #24487

[rsimha]

Running time of gulp dist without parallel invocations of closure is ~11 min, which is not too different from earlier. I'd like to merge this and see what effect it has on the stalled builds in #24487 we're trying to fix.

[rsimha]

Dusted off this old PR and made a couple of changes.

• Reduce parallelism only for multi-pass build (this is where parallelism might have been causing a stall)
• Fix minification step timing bug

Adding @danielrozenberg for review