Schema: Support database schema updates

Schema: Support database schema updates

See schema/README.schema, introduced in this patch, for the design.

Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
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v4: Add comment about test db safety catch.

v3: Fix spurious message from ./mg-schema-updates apply.
    Fix grammar error in README.updates.

v2: Slight increase schema update name length format.
    Docs fixes:
    Change erroneous `three' to `four'.
    Change `state' to `status' throghout.
    Explain scope of <status>.
    Sort out (and renumber) `Update order for Populate-then-rely'.
    Sort out "Statuses" explanations.
    Encourage use of DML update, rather than ad-hoc scripts,
     for populating new columns.