[gracolly] Set `spawn' method as default multiprocessing start method

Signed-off-by: Anton D. Kachalov <mouse@yandex-team.ru>
[fsmgracoll/modbus] Add `_send_register' and `_send_bits' functions to operate over whole value and separate bits.

Signed-off-by: Anton D. Kachalov <mouse@yandex-team.ru>
[cfg/metrics.py] Update example to follow new modbus' syntax

Signed-off-by: Anton D. Kachalov <mouse@yandex-team.ru>