dns_message_gettempname() allocates a dns_fixedname_t, which
includes a buffer and an offsets vector, so it is not necessary to
separately allocate a scratch buffer or a fresh offsets vector.
When parsing a message, owner names are deduplicated to collect
records into RRsets. This can require a lot of name comparisons.
Add a place to keep a hash value inside a dns_fixedname_t, and use
it to avoid comparisons.
Together these speed up a large zone transfer by about 2.5%
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