Fix return value of dt_conf_get_int64 to be int64_t

No harm done, this function isn't used anywhere yet.
Use dt_conf_get_int64 to read cache_memory
Extend the upper limit for the cache size to 4Gb.

This will let dt cache around 131000 compressed thumbnails.
Fix support for uint32_t value in preferences.
Merge pull request #300 from darktable-org/extend-cache-upper-limit

Extend the upper limit for the cache size to 4Gb.
Revert "Fix support for uint32_t value in preferences."

This reverts commit a2048b8e8cb062748129277d29b98af126887569.

This "fix" is bogus and probably does the opposite of what was intended.
And it breaks intended behaviour.
Make magic constant (4gb) more readable