Refactor DDLogMessage, DDLoggerNode and internal methods

Trying to modernize and keep DDLog contents to the minimum necessary.

Main changes:

- Remove superfluous `log` prefixes from properties.
- Use native `NSString` objects where possible and use less C code.
- Modernize code.

To do:

- [x] Update frameworks and demos.
- [x] Update DDLegacy.h.