Move logs from array to dedicated table

This avoids the append problem which bloats the database to 200+ gb in a
month and shrinks again to 5g with a full vacuum

migrations/000022_create_logs_table.down.sql

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+DROP TABLE IF EXISTS logs;

migrations/000022_create_logs_table.up.sql

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+CREATE TABLE IF NOT EXISTS logs(
+   id bigserial PRIMARY KEY,
+   task_id bigint REFERENCES tasks(id) NOT NULL,
+   message text
+);

migrations/000023_create_index_logs_table_task_id.down.sql

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+DROP INDEX logs_task_id;

migrations/000023_create_index_logs_table_task_id.up.sql

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+CREATE INDEX logs_task_id on logs(task_id);

migrations/000024_migrate_logs.down.sql

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+/* migration is irreversable */

migrations/000024_migrate_logs.up.sql

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+INSERT INTO logs (task_id, message)
+SELECT id as task_id, l as message
+FROM tasks, unnest(log) l ORDER BY task_id ASC;