Make zip cache configurable

A simple in-memory cache was introduced in !351 (merged) and !364 (merged) with some hard-coded values. These should be configurable.

Proposal

• Add configuration changes flag to Pages
  • Add a zipCacheConfig type and flags:
  • zip-cache-expiration
  • zip-cache-cleanup
  • zip-cache-refresh
  • zip-open-timeout
  • zip-cache-dataoffset-size
  • zip-cache-dataoffset-prune-count
  • zip-cache-readlink-size
  • zip-cache-readlink-prune-count
• Add documentation
• Add flag to omnibus

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The following discussion from !351 (merged) should be addressed:

• @jaime started a discussion: (+2 comments)

  question: are these values good enough? Should we make them configurable in future iterations?