This commit adds "isc_dnsbuffer_t" object implementation, a thin
wrapper on top of "isc_buffer_t" which has the following
characteristics:
* provides interface specifically atuned for handling/generating DNS
messages, especially in the format used for DNS messages over TCP;
* avoids allocating dynamic memory when handling small DNS messages,
while transparently switching to using dynamic memory when handling
larger messages. This approach significantly reduces pressure on the
memory allocator, as most of the DNS messages are small.
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