pack: evict objects from the cache in groups of eight

This drops the cache eviction below libcrypto and zlib in the perf
output. The number has been chosen empirically.
pack: evict all of the pages at once

Somewhat surprisingly, this can increase the speed considerably, as we
don't bother trying to decide what to evict, and the most used entries
are quickly back into the cache.
Merge pull request #1244 from carlosmn/pack-evict

A comparison of eviction algorithms for the delta base cache