FileStore.messageIndex is not cleared on reset

FileStore.messageIndex is not cleared on reset. As a result, get(int, int, Collection) may include "junk" entries for sequence numbers that used to be known to the store but are no longer. You can usually work around this by being careful not to peek beyond the current sequence number.

Suggested fix would be to add
messageIndex.clear()
to the top of initializeMessageIndex()

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