Support extended time zones without invalidating the cache

[omus]

The test/tzdata/build.jl tests compute time zone transitions into the far future which currently requires us to override the compiled time zones and reload the cache. It would be better if this interface supported computing using these time zones all from within memory instead of having to write to disk. This would allow some test code to be cleaner as well as avoid issues with other Julia processes writing different data to the same scratch space.

Ref: #441 (comment)