shell: Refactor shell_history to use less RW memory

Shell history module reworked to use ring buffer for storing
commands. Dedicated buffer is used to story all command lineary.
History capacity is in bytes not in number of entries, e.g.
many short commands can be stored or few long (depending on
CONFIG_SHELL_HISTORY_BUFFER).

Removed implicit command null termination from shell_history and
added it to shell after fetching command line from the history.

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