timers: cross JS/C++ border less frequently

This removes the `process._needImmediateCallback` property
and its semantics of having a 1/0 switch that tells C++ whether
immediates are currently scheduled.

Instead, a counter keeping track of all immediates is created,
that can be increased on `setImmediate()` or decreased when an
immediate is run or cleared.

This is faster, because rather than reading/writing a C++ getter,
this operation can be performed as a direct memory read/write via
a typed array. The only C++ call that is left to make is
activating the native handles upon creation of the first
`Immediate` after the queue is empty.

One other (good!) side-effect is that `immediate._destroyed` now
reliably tells whether an `immediate` is still scheduled to run or not.

Also, as a nice extra, this should make it easier to implement
an internal variant of `setImmediate` for C++ that piggybacks
off the same mechanism, which should be useful at least for
async hooks and HTTP/2.

Benchmark results:

    $ ./node benchmark/compare.js --new ./node --old ./node-master-1b093cb9 --runs 10 --filter immediate timers | Rscript benchmark/compare.R
    [00:08:53|% 100| 4/4 files | 20/20 runs | 1/1 configs]: Done
                                                         improvement confidence      p.value
     timers/immediate.js type="breadth" thousands=2000      25.61 %         ** 1.432301e-03
     timers/immediate.js type="breadth1" thousands=2000      7.66 %            1.320233e-01
     timers/immediate.js type="breadth4" thousands=2000      4.61 %            5.669053e-01
     timers/immediate.js type="clear" thousands=2000       311.40 %        *** 3.896291e-07
     timers/immediate.js type="depth" thousands=2000        17.54 %         ** 9.755389e-03
     timers/immediate.js type="depth1" thousands=2000       17.09 %        *** 7.176229e-04
     timers/set-immediate-breadth-args.js millions=5        10.63 %          * 4.250034e-02
     timers/set-immediate-breadth.js millions=10            20.62 %        *** 9.150439e-07
     timers/set-immediate-depth-args.js millions=10         17.97 %        *** 6.819135e-10

PR-URL: https://github.com/nodejs/node/pull/17064


Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Minwoo Jung <minwoo@nodesource.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>