Introduce new method exists as replace for old get_meta functionality

As get_meta no longer checks if the node exists, this is a new method
to check safely if a node exists.
Implement exists in all connectors (except SOAP::Lite) and add tests
Add stubs for get_meta and exists in SOAP::Lite - not working

Class implements hash and list and does not have any confg to determine
the type - need to be split into two classes or invent config value.
Minor fixes and improvements
Change "return undef" to "return"