Resolve "Unit tests unter Windows"

See merge request !56

- add scopes to enforce destruction of FileWriter, i.e. close file
- only check file size after writing the complete file

Something something string literals on Windows...

The singleton instances of ConfigurationManager and ConnectionManager
were stored as static variables in the static GetInstance methods.

This causes weird errors on Windows where the singleton instance is not
initialized in time and remains empty. This could be due to
undefined behavior?

Moving them to the namespace level fixes the unit test errors.

Resolve "Windows: Linker errors with protobuf-generated code"

See merge request !54

Rename caosdb-cpplib to linkahead-cpplib

See merge request !55

Fixes issue #95

This makes handling linker errors on Windows related to the protobuf
sources easier to handle.

caosdb-client.json is still kept, but linkahead has precedence.

TODO: Add tests checking support for caosdb_client.json