Fully handle layer namespace

[ocourtin]

Right now several internal functions assume that layer name is unique, and don't care about related namespace (even if namespace is already retrieved in storage structure)

From userland this lead to be able to retrieve a layer even if namespace prefix or uri is wrong, and to not been able to handle several layers having same name in several namespaces.

Quite a big issue, as it imply code refactoring.

[ocourtin]

Still an open ticket as i still need to:

• handle namespace more strictly for property too (right now a property namespace prefix could be shared with other layers)
• Refactor ows_layer stuff to retrieve layer struct rather than to use buffer