SQL tables can be upgraded using:
  UPDATE config SET name = 'nid' WHERE name = 'uuid';

then for MySQL:
  ALTER TABLE pt CHANGE uuid nid INT NOT NULL;

or SQLite:
  ALTER TABLE pt RENAME TO old_pt;
  CREATE TABLE pt (rid INTEGER NOT NULL, nid INTEGER NOT NULL, state INTEGER NOT NULL, PRIMARY KEY (rid, nid));
  INSERT INTO pt SELECT * from old_pt;
  DROP TABLE old_pt;

This fixes a bug in backup mode where 'loid' was not updated.

This is a follow up of commit 1f6bbcc1
("Rewrite (inside protocol, to make it standalone) a simpler Enum type"),
which renumbered values of CellStates enums.

Replication is also fully reimplemented:
- It is not done anymore on whole partitions.
- It runs at lowest priority not to degrades performance for client nodes.

Schema of MySQL table is changed to optimize storage layout: rows are now
grouped by age, for good partial replication performance.
This certainly also speeds up simple loads/stores.

This optimizes storage layout on disk, because more recent entries are often
being accessed more.

This will also simplify implementation of incremental backups.

Storage API is changed so that backends are not forced to use the checksum to
index data.