We recently investigated performance issues on BPEL mapper and BPEL diagram and found that one of the major reasons for
performance leaks is located in ModelSource lookup.

This problem is mostly related to IZ125937, but this time I want to extend the problem scope and provide more scenarios,
so I filed this new issue.

So,  2 places, where we have performance problems are:

1. Get XAM model by given file object.
2. Get file object by given XAM model

First one is called every time when reference resolver is trying to resolve cross-model reference. For example, this was
a main cause for IZ130061.

Second place is involved in scenarios, where we need to know read-only state of the model, such as enabling/disabling
menu items, responding to DnD events and showing editors in property sheets. So, having slightest delays here also
decreasing UI responsiveness a lot. 

So, basic solution I see here is again in area of improving ModelSource Lookup's performance. Profiling shows that now
it suffers from timing of _getDocument() method. This method is called when we trying to query ModelSource lookup for
ANY type of object, such as FileObject, DataObject, ModelAccessProvider. 

In issue 125937 we tried to avoid this by caching Document returned by _getDocument() method, but this created a
regression around xml source and model synchronization. New proposed solution is to create lookup which will call
_getDocument() ONLY WHEN lookup is queried for corresponding object type. All other objects, stored in lookup are
constants and fixed lookup can be used to store them. This can be achieved with using AbstractLookup together with
InstanceContent.Convertor for Document-type content. This lookup can be combined using ProxyLookup with fixed lookup,
storing FileObject, DataObject and other constants. This should greatly reduce time required to query ModelSource lookup
for FileObject and as a result, improve ModelSource cache performance.

To provide further performance gains, we can add new utility method in ModelFactory, which will return Model by given
FileObject. Additional caching of FileObject->Model pairs is requied to avoid querying ModelSource lookup completely.
This utility method should try to find already loaded Model by given file object, and if not found, construct new
ModelSource and call old method. This will also simplify a client code, because i found about 4 similar methods doing
this only in BPEL editor module. All these helper methods can be removed in favor of this new ModelFactory method.