Support for master data sequences

[BEagle1984]

Support a new type of sequence to deal with master data replication. Some headers will be used to delimit the dataset.

It should ideally be possible to publish a collection of messages and have the Silverback producer automatically add the necessary headers. The consumer should recognize the dataset and create an ISequence to deal with it as a unit of work, where the subscriber recognizes when all messages have been received (subscribing to the IMessageStreamEnumerable, that will complete when the entire set has been consumed).

The headers should be as follows.

Header	Description
x-dataset-last	Marks the last dataset element in this partition.
x-dataset-first-offset	Starting offset of the dataset in this partition. Present in all records except the first one of each partition.
x-dataset-version	Optional. Monotonic increasing number identifying the dataset version.
x-dataset-index	Sequence number, within a version. Starts with zero.
x-dataset-count	Optional. Total number of records.