Add repack mode

[jberkus]

Depends On: #4, #6, probably others.

Add mode to flexible_freeze to opportunistically pg_repack severely bloated tables. This would be very useful, but suffers from a number of challenges:

• currently pg_repack is not designed to be called from another program. It's output depends on being read by a human on stderr. pg_repack would need an API, which might not be feasible.
• there's no safe way to cancel a pg_repack if the time limit runs out. So we'd need to estimate the time required for a repack before starting.
• since pg_repack will cancel competing transactions, do we really want to run it unattended?
• also, pg_repack can use up disk space, so we'd need a way to check available disk space and refuse to proceed if it's not at least (table_size * 2). However, a platform-agnostic way to check available disk space would be complicated.