PHPLIB-1185: Add new methods to get database and collection instances

[alcaeus]

PHPLIB-1185

This PR adds the following new methods:

• Client::getDatabase, changed Client::selectDatabase and Client::__get to proxy to this method
• Client::getCollection, changed Client::selectCollection to proxy to this method
• Database::getCollection, changed Database::selectCollection and Database::__get to proxy to this method

Since they are widely used, I did not add any deprecation warnings, nor did I add any language to the documentation to indicate that they are deprecated. I left the tests as they are, as the old methods were changed to proxy to the new methods, so we are implicitly testing them. I can duplicate the existing tests, but I don't think this is necessary.

As a follow-up to this PR, we should update the documentation to use the new get(Database|Collection) methods, starting in 2.0 (as we only maintain a single documentation for 1.x, so users would have to figure out which method is correct for their version).