Some initial work to get snapshots going.
Changes required, as need to snapshot through the Bookie to ensure that
there is no race between extracting the Bookie's in-memory view and the
Penciller's view if a push_to_mem has occurred inbetween.
A lot still outstanding, especially around Inker snapshots, and handling
timeouts
Two aspects of pushing to the penciller have been refactored:
1 - Allow the penciller to respond before the ETS table has been updated
to unlock the Bookie sooner.
2 - Change the way the copy of the memtable is stored to work more
effectively with snapshots wihtout locking the Penciller any further on
a snapshot or push request
Add test to show inker rolling journal. to achieve needs to make CDB
size an option, and also alter the manifest sorting so that
find_in_manifest actually works!
Standardise on record definitions between modules to make easier - then
add functionality to pushing to penciller as bookie would do. Some
initial manual testing of this seems OK.
