Common.Logging + other logging work

- Killed the internal NpgsqlEventLog, set up Common.Logging instead
- Delocalized logging messages - all logging is in English to help
  analyzing logs provided by users etc.
- Log messages now have level to allow filtering. Existing messages
  categorized as DEBUG or TRACE.
- Removed ultra-high-resolution logging messages to help readability
- Unit tests setup NLog with Common.Logging
Reorganized localization

- Grouped all localized strings into per-language L10N resx files, stored in
  a folder, instead of the previous resx-per-type system
- Code now references localized strings statically rather than via the ResourceManager
- Reminder: localization for logging messages was removed in the previous commit