MINOR: cli: add a new "wait" command to wait for a certain delay

MINOR: cli: add a new "wait" command to wait for a certain delay

This allows to insert delays between commands, i.e. to collect a same
set of metrics at a fixed interval. E.g:

  $ socat -t20 /path/to/socket <<< "show activity; wait 10s; show activity"

The goal will be to extend the feature to optionally support waiting on
certain conditions. For this reason the struct definitions and enums were
placed into cli-t.h.