Count all UDP compression attempts and failures, including in Semi-Reliable UDP.

The new GNR_UDP_COMPRESSION_ATTEMPTS counter tracks all the UDP compression
attempts we make, either in the semi-reliable UDP layer, or explicitly when
gmsg_split_to_deflated_pmsg() is called.

Likewise, the GNR_UDP_LARGER_HENCE_NOT_COMPRESSED counter holds the total amount
of messages we could not compress.