Fix bug where ctime was not set right

The implicit scope for `tipper`/`tippee` is the current select, which
turns `where tipper=tipper` into a no-op, so that we had been pulling
the ctime for the tip record that happened to be first among all tips
when the sort order was undefined. We were setting ctime for all new tip
inserts to this single undefined value.

This commit fixes the problem by explicitly scoping tipper/tippee in the
inner where clause to the current_tips table in the parent select.