The goal of this ticket is to add a timestamp to DatabaseVersion IDL. In order to do that we will:

1. Rename the class defined in the DatabaseVersion IDL to DatabaseVersionBase.
2. Create a new DatabaseVersion class that works as a facade of DatabaseVersionBase.
   1. This new class will inherit from DatabaseVersionBase.
   2. We will try to avoid exposing its constructors. Instead of that, we will offer a factory that will hide the epochs/timestamps. We should be avaible to construct it from a BSON and probably default construct them.
3. We can get rid of database_version_helpers.h/database_version_helpers.cpp.