Connection::getParams() leaks implementation details

[morozov]

Q	A
BC Break	no
Version	2.9.3

The fact that the Connection::getParams() method exists makes it impossible to make any changes in the structure of the connection $params array w/o breaking the API. In #3568, we already removed ::getHost(), ::getPort(), ::getUsername() and ::getPassword().

We need to see if it's possible to get rid of the method entirely or at least reduce its usage and mark it as @internal.