reftable: improvements for the `binsearch()` mechanism

This patch series contains various improvements for the binsearch() mechanism:

• Callsites are refactored to make it more obvious what exactly they are doing. This mostly involves improved names for callback functions.
• Error handling is improved such that binsearch() knows to abort when the callback returns a negative value.
• Error handling for binary searching over restart counters is fixed so that we check for errors at the correct point in time.
• Decoding of record keys is refactored when searching over restart counters to not require allocations anymore.

Closes #279 (closed).