process: slight refinements to nextTick

Remove length prop on NextTickQueue class. We technically don't need to
keep track of the length of the queue in two places as we already have
tickInfo doing that work (between the index & the length we have enough
data for everything).

Store asyncId in a const within the _tickCallback function. Accessing
Symbol properties seems to be quite a bit more expensive than string
keys so this actually has a decent performance impact.

PR-URL: https://github.com/nodejs/node/pull/17421


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>