TS-2627: mgmt condense duplicated socket read code

Rather than copy the code to read a fixed amount from a socket,
make a helper function and call it.
TS-2628: traffic_line: print restart status after setting a value

The API that sets configuration values reports whether Traffic
Server needs to be restarted or reconfigured. Print this information
to the user so that it is clearer what the next step should be.
This closes #60