Addressing MongoAscoltatore 'close' bug

[mcollina]

There are some spurious errors when closing a MongoAscoltatore.

This pull-request rewrites entirely the 'close' method, trying to solve the issue once and for all.

@davedoesdev could you please try it on your machine?

[davedoesdev]

Tests pass here.
Mongo stuff is much better!
Regarding the commented out test. I agree it didn't really test much but I'm relying on passing a function to ascoltatori.build. I wonder if a good way to test it would be to pass one of the existing ascoltatores as a function with some settings?

[davedoesdev]

Ah, I think I can modify can_be_used_by_ascoltatori.js to incorporate that so go ahead and merge this pull request. I'll do another one for passing type as a function.