N-API: Reuse ObjectTemplate instances

V8 caches and does not subsequently release `ObjectTemplate` instances.
Thus, we need to store the `ObjectTemplate` from which we derive object
instances we use for `napi_wrap()` and function/accessor context in a
persistent in the `napi_env`.

https://github.com/nodejs/node-addon-api/pull/70#discussion_r124998408

Backport-PR-URL: https://github.com/nodejs/node/pull/19447
PR-URL: https://github.com/nodejs/node/pull/13999


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.co>
Reviewed-By: Hitesh Kanwathirtha <digitalinfinity@gmail.com>