Allow overriding current DB escape hatch

[jwoertink]

In #1089 we now have a way to specify different Read and Write databases. One case I wasn't able to figure out a good solution for is when you might need to override this value on the fly for specific edge cases.

Imagine something along these lines:

# temporarily switch to a different DB only for these read/write
# actions.
Avram.use_database(CustomDB) do
  user = UserQuery.find(1)
  SaveUser.update!(user, whatever: true)
  SoftDeleteUser.delete(user)
end

# uses the normal ReadDatabase
UserQuery.find(2)

I'm thinking that in order to do this, we may need to refactor how the databases are setup. At the moment we use a class as the definition:

def self.read_database : Avram::Database.class
  ReadDatabase
end

But this may need to become an instance....

getter read_database : Avram::Database do
  ReadDatabase.new
end

If we went with instances everywhere, then we could easily do something like

def self.use_database(db) do
  old_db = current_db
  current_db = db
  yield
  current_db = old_db
end

This will need a bit more planning though to make sure it will be thread safe so when multithreadding comes we're ready.