Use [] rather than find on map so we only lookup once.

(Needs new type for the entries in the cache so we can tell if they're
just constructed.)
Invert meaning of "-n"

The extra debug info is useless for the mo
Revert change of reading long strings

Breaks if the 32-byte chunk extends past the valid page when the string
is shorter than 32 bytes. Really need to make the version in CacheReader
page-aware, rather than just delegating to Reader::readString