BUG/MINOR: mworker/cli: fix set severity-output support

BUG/MINOR: mworker/cli: fix set severity-output support

"set severity-output" is one of these command that changes the appctx
state so the next commands are affected.

Unfortunately the master CLI works with pipelining and server close
mode, which means the connection between the master and the worker is
closed after each response, so for the next command this is a new appctx
state.

To fix the problem, 2 new flags are added ACCESS_MCLI_SEVERITY_STR and
ACCESS_MCLI_SEVERITY_NB which are used to prefix each command sent to
the worker with the right "set severity-output" command.

This patch fixes issue #2350.

It could be backported as far as 2.6.