Important session enviroment variables pushdown, e.g. time_zone, client_set_connection

MCS functions output differs from MDB in some cases b/c there are certain session variables that affects the functions output, e.g. JSON_QUOTE.
As of now MCS pushes down time_zone value in a very clumsy manner. We need to replace a single int value with a structure that is easy to extend. This structure will pushdown all the relevant session variable values down to PP and EM runtimes where FuncExp/ParseTree classes are used.