Updated persistence to work with latest mkdirp. Pulled events back in…

[bnielsen1965]

…to Datastore to fix multiple bugs due to missing emit and on functions. Fixed db test that was failing due to unhandledRejection exception.

This was originally meant to resolve breakage caused by the latest mkdirp but several tests also no longer worked so I added in those fixes as well.

[bnielsen1965]

@pi0 Can you update the travis settings on your repo to use travis.com in place of travis.org? They are ending the travis.org domain use and builds will be slow or potentially not work on the .org domain anymore.

Also, if this pull request passes after switching to travis.com can you do a review to see if you can accept it?

The latest mkdirp package had changes that broke nedb-core and it will require nodejs 10 or higher.

Thanks,
Bryan

EDIT: travis check completed and failed, looking into the issue...

EDIT 2: The problem is that travis is building the last master commit not the PR so the build fails as expected on the mkdirp. The PR fixes this but is not being tested by travis.

Are PR builds enabled in the settings? If I run tests locally on the PR they all pass.