BUG/MINOR: samples: fix unnecessary memcopy converting binary to string.
MINOR: sample: add a few basic internal fetches (nbproc, proc, stopping)

Sometimes, either for debugging or for logging we'd like to have a bit
of information about the running process. Here are 3 new fetches for this :

nbproc : integer
  Returns an integer value corresponding to the number of processes that were
  started (it equals the global "nbproc" setting). This is useful for logging
  and debugging purposes.

proc : integer
  Returns an integer value corresponding to the position of the process calling
  the function, between 1 and global.nbproc. This is useful for logging and
  debugging purposes.

stopping : boolean
  Returns TRUE if the process calling the function is currently stopping. This
  can be useful for logging, or for relaxing certain checks or helping close
  certain connections upon graceful shutdown.
MINOR: samples: adds the bytes converter.

bytes(<offset>[,<length>])
  Extracts a some bytes from an input binary sample. The result is a
  binary sample starting at an offset (in bytes) of the original sample
  and optionnaly truncated at the given length.
MINOR: samples: adds the field converter.

field(<index>,<delimiters>)
  Extracts the substring at the given index considering given delimiters from
  an input string. Indexes start at 1 and delimiters are a string formatted
  list of chars.