IMessageStore extended with method SetAndIncrNextSenderMsgSeqNum, resolves issue #987

[asmeisne]

The IMessageStore interface is extended with a new method SetAndIncrNextSenderMsgSeqNum with a default implementation to ensure backwards compatibility. This new method now is used in Session.Persist() to allow execution of methods IMessageStore.Set() and IMessageStore.IncrNextSenderMsgSeqNum() in one transaction. Two unit tests for FileStore and MemoryStore are added.

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