The old cache had problems:
     * It needed to be manually preloaded. (It didn't remember any
       address you didn't tell it to remember)
     * It was AF_INET only.
     * It looked at its cache even if the sandbox wasn't turned on.
     * It couldn't remember errors.
     * It had some memory management problems. (You can't use memcpy
       to copy an addrinfo safely; it has pointers in.)

This patch fixes those issues, and moves to a hash table.

Fixes bug 11970; bugfix on 0.2.5.1-alpha.