Several improvements and suggestion

[Kivol]

Hi, thanks to your effort, I was able to build my custom version of node 14.x lambda layer and docker image. I built it from your fork(This PR: lambci#25 ), and I'd like to share some of my changes for the completeness of this layer.

The changes are:

• Upgraded to 14.15.4 which is the latest LTS and includes security patch
• I moved NODE_OPTIONS to header on bootstrap.c.
• I modified test.sh to remove unnecessary NODE_MODULE_LOADER_TYPE environment variable.
• (This is more like a suggestion) I rollback the getHandler() of bootstrap.js to ensure the backward compatibility of succeed/fail/done callback and test code.

Please feel free to give any comment!

Thanks,
Kivol