Add support for nullable annotations

[FreeAndNil]

Public classes and interfaces should get nullable annotations so that callers will be warned when passing null values.

List of breaking changes

#129 - Part 4 of nullable and modernization

• interface log4net.Appender.AppenderCollection.IAppenderCollectionEnumerator was replaced by IEnumerator<IAppender>
• the signature of log4net.Appender.AppenderCollection Add and Remove changed (for implementing ICollection<IAppender>)
  • from public virtual int Add(IAppender item) to public virtual void Add(IAppender item)
  • from public virtual void Remove(IAppender item) to public virtual bool Remove(IAppender item)
• log4net.Appender.MemoryAppender.m_eventsList (protected field) was changed from ArrayList to List<LoggingEvent>
  • for getting m_eventsList.SyncRoot you need to cast it: ((ICollection)m_eventsList).SyncRoot