Initial draft making LZ4 the default compression method.  Now,
compression=on means use LZ4 as compression method if the pool
have LZ4 activated, and LZJB if pool is using old format.  Also,
metadata is now using LZ4 as compression method by default.

Notes to my self based on talk with mm@:

 - Need to check how the LZ4 feature flag is activated.  E.g. what
   happens if a new pool is created, do we need to explicitly do
   the feature activation?

 - Do we need a sysctl variable for this?  Do we want to make it
   possible for users to tweak the method themselves?