Explicitly WARN about pausing/throttling and binlog expiration

[rlowe]

<captaineyesight>Might I suggest putting some kind of bolded note about only being able to throttle as long as binary logs hang around. I know it should be 100% obvious for everything else written. I just don’t want someone crying that they paused their schema change for 10 days and they only saved 5 days of binary logs and that somehow is the tools fault.

<rlowe> I'd go one step further and warn interactively when they pause where possible.`

[shlomi-noach]

Closed by #112