Give pool a save_state() and a load_state() method
Give pool a save_state() and a load_state() method

load_state() is just a rename of read_metadata().
write_metadata() is split into write_metadata() and the lower-level
save_state() which takes time and data as arguments. This split
should isolate testing a bit better.

Signed-off-by: mulhern <7b51bcf507bcd7afb72bf8663752c0ddbeb517f6@redhat.com>

trgill/stratisd,trgill/stratisd,mulkieran/stratisd,mulkieran/stratisd,stratis-storage/stratisd,stratis-storage/stratisd,stratis-storage/stratisd