MINOR: master/cli: support bidirectional communications with workers

MINOR: master/cli: support bidirectional communications with workers

Some rare commands in the worker require to keep their input open and
terminate when it's closed ("show events -w", "wait"). Others maintain
a per-session context ("set anon on"). But in its default operation
mode, the master CLI passes commands one at a time to the worker, and
closes the CLI's input channel so that the command can immediately
close upon response. This effectively prevents these two specific cases
from being used.

Here the approach that we take is to introduce a bidirectional mode to
connect to the worker, where everything sent to the master is immediately
forwarded to the worker (including the raw command), allowing to queue
multiple commands at once in the same session, and to continue to watch
the input to detect when the client closes. It must be a client's choice
however, since doing so means that the client cannot batch many commands
at once to the master process, but must wait for these commands to complete
before sending new ones. For this reason we use the prefix "@@<pid>" for
this. It works exactly like "@" except that it maintains the channel
open during the whole execution. Similarly to "@<pid>" with no command,
"@@<pid>" will simply open an interactive CLI session to the worker, that
will be ended by "quit" or by closing the connection. This can be convenient
for the user, and possibly for clients willing to dedicate a connection to
the worker.