Fixed bad PlatformTarget when building unit tests in Release mode
Unit tests: set default timeout to 15 sec for the test db
A little bit of refactoring on the LINQ providers

- removed the FdbQueryNodeType property, which is not needed
- moved provider specific code in FoundationDB.Linq.Providers namespace
- cleaned up the usings
Quick hack to display a prominent error message in Debug when attempting to reuse an existing process
Simplified a bit the experimental LINQ provider (still a work in progress)
Fixed LINQ index lookup tests
Directory Layer: added overrides that take a subfolder name on FdbDirectorySubspace
Added more info to exceptions descriptions