stream: skip idle webstreams start and watchers

A source without pull() has nothing observable left to do in its
post-start step, since the started flag only gates calls into the pull
algorithm. Set the flag right away instead of from a microtask, so a
push-style ReadableStream allocates neither the closure nor the task
and is not kept alive until the next microtask checkpoint.

pipeTo and tee hold the only references to their reader and writer, so
their [[closedPromise]] records are never observed as promises. Install
the watchers as the records themselves, as pipeTo's ready hook already
does, instead of materializing a promise plus reaction per side, and
hand the erroring/release probes one shared pending promise. The tee's
cancel promise is likewise materialized by the first branch cancel.

Microtask ordering is unchanged: each hook enqueues its watcher at the
position the promise reaction would have had.

node benchmark/compare.js --runs 20 over benchmark/webstreams (46 rows,
all others within the confidence interval):

webstreams/creation.js kind='ReadableStream'               ***  +172.40%
webstreams/creation.js kind='ReadableStream.tee'           ***   +20.47%
webstreams/creation.js kind='ReadableStreamBYOBReader'     ***   +15.66%
webstreams/creation.js kind='ReadableStreamDefaultReader'  ***   +14.53%
webstreams/lifecycle.js kind='pipe-to' (40 runs)           **    +10.98%

Signed-off-by: Matteo Collina <hello@matteocollina.com>

Author: mcollina