Dangling AVP references in readerBufferPool

[jefftt]

It is possible for AVPs to be overwritten or their memory dropped while still being processed, if the underlying readerBuffer is reused or the underlying sync.Pool removes it respectively. This is because for some avp datatypes are references into the original buffer, and the original buffer is reset and put back into the readerBufferPool right after the message is decoded [ref].

Affected datatypes are:

• Address
• Grouped
• Ipv4
• Ipv6
• Unknown

Quick fix would be just to copy out of the original read buffers for these types