[hotfix/analytics] more robust buffer

[thibaultcha]

• Fix a critical issue where the buffer was not flushing itself when
  reaching the max size (in bytes).
• Better logging in case of refused batch (number of items, size of the
  batch).
• Dropping of the batch if it is refused by the socket server (HTTP
  400).
• If a batch is not accepted for another reason, it is now pushed to the
  end of the queue instead of keeping blocking it.
• Dropping single ALF objects that exceed the max bytes size by
  themselves.
• Tests for size limit reaching.