Better to have all formatting on the client side, so we format the
header and the reponse values in the same place. (Also no need to
send this data over the wire as it's always the same).

PB now accommodates "lpa" and "lpr" responses as well as "af ls"

OptionStrList takes a filename as its value. It reads the file into a
list of strings in the protobuf. On the server side, the values are
unpacked and made available as std::vector<std::string>.

For example implementation, see "cta-admin repack add"

Integrates XRootD logging

Default now set to 10s. Note that XRD_TIMEOUTRESOLUTION must be set
<= 10s, otherwise timeout defaults to 15s.

In the case of a Stream response, XRootD allocates the memory and
manipulates the buffer pointer. In the case of a Data response, we need
to allocate the buffer ourselves. In both cases, the buffer is treated
in ProcessResponseData(), which does not care how the buffer was filled.

This requires a bit of extra logic to make sure the buffer is
deallocated for Data responses but not Stream responses. I have done
this using a unique pointer to allocate a char[] for Data responses.
and char *m_buffer_ptr points to the memory allocated.

We never delete m_buffer_ptr as memory allocation is handled either
by the unique_ptr or by XRoot and we don't care which.