Investigate whether clustersCheckinsTable can use a *Timestamp

[arschles]

it's currently using a string because at the time, it looked like gorm couldn't handle (database/sql/driver).Valuers. I've found, in #105, that it can, however. Investigate whether it can use a *Timestamp instead (which itself is a Valuer)

[arschles]

In #123 (specifically ee4613e), the versionsTable uses a Timestamp (no pointer), so this should be possible

[arschles]

I'm moving this to RC1

[mboersma]

@arschles have we implemented the pointer change described here?