Fix timeout units and index stats width

[tanmay-9]

The --timeout of qlever start is passed on to the server, which accepts a unit (ns, us, ms, s, min, h). The commands that need the timeout seconds as a plain number, however, obtained it by cutting off the last character of the string, so anything but seconds broke them.

With this change, --timeout is validated when it is parsed, so an unusable value is rejected right away, with a message naming the units that are accepted, instead of failing later in whichever command happens to need a number. The conversion to whole seconds now happens in one place for all commands, and a sub-second timeout becomes one second rather than zero, so it stays a timeout.

On the side, the headings of qlever index-stats are now padded to the width of the longest heading (at least the previous 25 characters, so the QLever output is unchanged) and to the same width in the time and the space part, so that the colons still line up for engines whose phases and file groups have longer names.