Add support for tracking manual exceptions within transactions

[LALOPSB]

Summary

Manual errors tracked on transactions (i.e custom validations) were not tracked.
This happened because whenever the transaction failed, it was rollbacked, and as the error tracking depends on the persisting of a record on the db, that record was also rollbacked along the transaction that contained it.

Now we've included a way of allowing the ExceptionHunter::Error to be persisted as a part of a new thread not implicated on the transaction itself, so it (alone, the rest of the transaction will) won't be rollbacked.

This new thread is only created if the tracking happens within a transaction.

Examples:

class Payment < ActiveRecord::Base
  validate :is_friend

  belongs_to :friend, class_name: 'User', foreign_key: :friend_id
  belongs_to :sender, class_name: 'User', foreign_key: :sender_id

  private

  def is_friend
    unless (sender.friends.include?(friend))
      errors.add(sender.username, 'you can make payments only to friends.')
      ExceptionHunter.track(ArgumentError.new('Error tracked within transaction'))
    end
  end
end 

Before:

The whole transaction is rollbacked and no records have been persisted

Now:

Only the outer transaction is rollbacked and the ExceptionHunter::Error remains persisted.

[brunvez]

Can you add an example in the description? It would help people looking at this in the future

[brunvez]

Shouldn't we join it immediately?