auparse: add new AuditMessage.Parse method to allow preallocated messages

[jeffmahoney]

This commit moves message parsing into a new AuditMessage.Parse method. The existing Parse() function allocates an *AuditMessage and calls the method, keeping the API consistent.

This allows the caller to use a pool of preallocated messages to prevent GC churn, which can cause dropped messages when there is a lot of audit activity.

[djoreilly]

I don't understand how this patch allows for reduced allocations. AuditMessage.RawData is a string, how can it be preallocated?