The new v3 applets don't use it, and eventually all applets will
use the new formats only.
This prepares the code for this, and moves trust, id_cache, and
root_fd to apk_ctx which is enough for v3 applets at this time.
The generic code is made to not initialize apk_database if it's
not needed.
This is a flat buffers inspired format that allows fast
mmaped access to the data with low overhead, signature support
and relatively good forward support.
Timo Teräs