osd: generalize coll_t to a string

osd: generalize coll_t to a string

coll_t is now a string. META_COLL and TEMP_COLL are just constants now.

Now there is a constructor that takes pgid_t and snapid_t, rather than
factory methods. It's clear what that constructor does, so wrapping it
in factory methods should be unecessary.

Bump coll_t serialization version to 3. Implement decoding for the old
versions.

Signed-off-by: Colin McCabe <colinm@hq.newdream.net>