Update session functions to use apex_session

[martindsouza]

in 18.1 apex_session is a supported way to create and/or join (attach) a session. In our code detect if we're in 18.1. If so then use apex_session instead of our custom code.

@commi235 suggested using undocumented compilation variables which will simplify things

Ex:

$if wwv_flow_api.c_current >= 20160824
$then