Database: allow to mark an entry as static

[MatthieuDartiailh]

The idea here would be to allow to specify that once written an entry won't change during a measurement. This would obviously apply to Definitions and any value resulting of the evaluation of static fields. Having this capability would allow some optimisation in particular regarding the saving of data. For example HDF5 could take adavntage of knowing the size of the final array to allocate properly the data, or generate enough frames.
The exact API needs some reflexion.
Another advantage would be for providing more accurate infos to the monitors by letting them know that some loops are static and that it is safe to estimate the time before completion.