Avoid setInterval keeping node process alive

[runeh]

No description provided.

[coveralls]

Coverage remained the same at 82.123% when pulling d4b183b on runeh:patch-1 into a53f150 on valery-barysok:master.

[coveralls]

Coverage remained the same at 82.123% when pulling d4b183b on runeh:patch-1 into a53f150 on valery-barysok:master.

[SimenB]

This fixes #27 without the user having to do anything. So in user code, one could do

const fileStoreOptions = {};
const store = new SessionFileStore(fileStoreOptions);

if (fileStoreOptions.reapIntervalObject) {
  fileStoreOptions.reapIntervalObject.unref();
}

I do think it should be the default behaviour, though