[bugfix] changing scope and do not close connection
[bugfix] changing scope and do not close connection

The current implementation opens and closes a connection on a single operation. Thus
there is no need to close the connection in the close handler.

Moved internals into the class scope.
Removed a duplicate method.
Moved connectionString into the class scope.
Use Date with parantheses.

Fixes https://github.com/db-migrate/node-db-migrate/issues/331
Fixes #2

db-migrate/mongodb