ConnectionHandler will not have any knowlodge of AR models now, it will
only know about the specs.
Like that we can decouple the two, and allow the same model to use more
than one connection.

Historically, folks used to create abstract AR classes on the fly in
order to have multiple connections for the same model, and override the
connection methods.

With this, now we can override the `specificiation_id` method in the
model, to return a key, that will be used to find the connection_pool
from the handler.