Remove private static cache from MongoDbHealthCheck

[adamsitnik]

MongoDbHealthCheck is maintaining it's private static cache of instances. It's wrong, as it can cause a memory leak (it won't ever be freed).

Moreover, it's can create instances of MongoClient. This is wrong, as it can lead into a situation when we have multiple instances of MongoClient that are connected to the same DB: one used by the app and another created and used by the health check. And we should have only one.

We should do the same as in #2040, #2116 and #2096