Log only partial response in Luxi when in debug mode

Currently, we log the entire response (at debug level) in the Luxi
replies. This is not a good idea; the logging library operates on
strings, and as such it will use huge amounts of memory: without debug
logging, a certain gnt-job list invocation uses 295MB (+RTS -s) and
2m35s time, when in debug mode, it's 1525MB and 48m!

So we make two changes:

- first, we switch from "show (pp_value a)" to "encode a", which
  generates a non-formatted string rather than a indented one
- second we log only the first 2000 characters; this should be enough
  to understand the first part of the response

We could go for higher, or for logging in batches (that would be
faster, as well).

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Helga Velroyen <helgav@google.com>