[@accounts/mongo] Use mongodb as peer dependency

[aslafy-z]

I seen that you were trying to keep compatibility with old mongodb driver versions but it looks that the mongodb dependency is locked to the latest version.
I think it should switch to a peer-dependency in order to let the user choose what version to use.

https://github.com/accounts-js/accounts/blob/master/packages/database-mongo/README.md#usage
https://github.com/accounts-js/accounts/blob/master/packages/database-mongo/package.json#L48
https://docs.npmjs.com/files/package.json#peerdependencies