shell: Improve handling of log messages

If burst of log messages was passed to the shell log
backend, it was likely that messages were lost because
shell had no means to control arrivals of log messages.

Added log message enqueueing timeout to the shell instance
to allow blocking logger thread if short-term arrival rate
exceeded shell capabilities.

Added kconfig option for setting log message queue size
and timeout in RTT and UART instances. Added section in
shell documentation which explains interaction between
the logger and shell instance acting as a logger backend.

Signed-off-by: Krzysztof Chruscinski <krzysztof.chruscinski@nordicsemi.no>