Mappedbus v0.2

Changes:

• Added a new field "rollback" to indicate whether a record has been abandoned. The reason it's needed is that a writer might crash after it has updated the limit field but before it has updated the commit field. To avoid this problem a new field next to the commit field called the rollback field has been added. The reader has a timeout for how long it will wait for the commit field to be set. When that time is reached the reader will set the rollback field and continue with the next record. The rollback field has precedence over the commit field, when the rollback field is set the record is always ignored by the readers.