tsfn: support direct calls to underlying napi_tsfn

[KevinEady]

Fixes: #556

[KevinEady]

The implementation for this is based off previous napi meeting discussion, see #556 (comment)

[KevinEady]

Like mentioned, this PR directly conflicts with #546 on the use of unique_ptr<napi_threadsafe_function> vs napi_threadsafe_function. It's a simple change to resolve, by changing *_tsfn to _tsfn ...

[mhdawson]

LGTM

[mhdawson]

Looks reasonable to me @gabrielschulhof can you take a look at as well?

[gabrielschulhof]

Suggested change

	- `data`: Data to pass to `call_js_cb` specified when creating the tsfn via
	- `data`: Data to pass to `call_js_cb` specified when creating the thread-safe function via

[KevinEady]

After a nasty fight between 500s from GitHub's commit suggestion API and various force pushes, this is comment has been updated 👍

[KevinEady]

Hi @mhdawson / @gabrielschulhof dunno what to do for this build failure:

https://travis-ci.com/nodejs/node-addon-api/jobs/253158482

Reading package lists...
Building dependency tree...
Reading state information...
E: Unable to locate package g++-4.9
E: Couldn't find any package by glob 'g++-4.9'
E: Couldn't find any package by regex 'g++-4.9'
apt-get.diagnostics
apt-get install failed
The command "sudo -E apt-get -yq --no-install-suggests --no-install-recommends $(travis_apt_get_options) install g++-4.9" failed and exited with 100 during .

[gabrielschulhof]

@KevinEady I restarted the job.

[KevinEady]

Hi @mhdawson just a reminder on landing this

[legendecas]

LGTM, just found some nits in test :P.

[mhdawson]

Landed as df75e08c2bae11b84ebbf5a1c6d54df8621b16fd