adds a multivals table

for columns that store multiple values (multi = true), we store a NULL
in the data_* table and put all the values in the multivals table. The
row column acts as a simple counter so values can be stored and
retrieved in the correct order.

db/latest.version

@@ -1 +1 @@
-2
+3

db/update0003.sql

@@ -0,0 +1,11 @@
+-- stores multi values for all tables, we might split this later into one multival
+-- table per data table
+CREATE TABLE multivals (
+    tbl NOT NULL,
+    colref INTEGER NOT NULL,
+    pid NOT NULL,
+    rev INTEGER NOT NULL,
+    row INTEGER NOT NULL,
+    value,
+    PRIMARY KEY(tbl, colref, pid, rev, row)
+);