Implement cron

Fixes #22

db/10-cron.sql

@@ -0,0 +1,11 @@
+CREATE TABLE cron (
+    jobName varchar UNIQUE NOT NULL, -- Application-recognizable name for the job
+    runinterval interval NOT NULL, -- Duration between runs
+		-- Last successful run - only gets updated if run is successful
+    lastSuccess timestamptz DEFAULT to_timestamp(0), -- Defaults to beginning of time
+		PRIMARY KEY(jobName)
+);
+
+INSERT INTO cron (jobName,runInterval) VALUES
+('processBans','30 minutes'),
+('processEmptyMetadata','1 day');