Concurrency issue in MsSqlStreamStore.AppendStream #542

Description:

MsSqlStreamStore.AppendStream's AppendToStreamInternal is not thread-safe when MaxCount has a value. For a high concurrent load relative to a low MaxCount, it may lead to all entries in the Messages table being deleted for the associated Stream, hence all data is lost.

Reproduction steps:

Let's assume that there are 2 messages associated to a stream, with MaxCount = 2, and there are 2 threads calling AppendToStreamInternal simultaneously. Hence, 2 new messages are appended to the stream at the same time. Then, CheckStreamMaxCount is called simultaneously, and the result of GetStreamMessageCount is 2 + 2 = 4 (2 already existing messages, and 2 newly added ones). Thus, the count of messages to be deleted is 2 for each thread. Since each thread tries to remove the 2 oldest messages, regardless, this might leave us with no messages at all, and all messages associated to the stream are lost.