MongoDB polling provider

[joaofbantunes]

Did some experiments this PoC.

Because MongoDB doesn't implement transactions in the same way relational databases do, and don't have something like advisory locks in MySQL or PostreSQL, need to implement a custom locking mechanism.

Note: this implementation will be targeted at the simplest installation, with a single primary replica. More complex installations with multiple primary replicas, like sharding, are outside the scope of this work.