Restrict the length of key and values when setting metadata

By setting very long keys and/or values over-and-over you can get the colord
daemon to crash with an out-of-memory error.
To prevent this, limit the length of the metadata key to 128 bytes and the
metadata value to 4k. Anything longer than that isn't really metadata...

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1012832