Use TIMESTAMP for Timestamp columns.

Probably too little too late. Maybe MySQL didn't support TIMESTAMP in ancient
history when we first wrote Depot? Or maybe I was dumb. Postgres does the right
thing, so now MySQL does too, FWIW.

src/main/java/com/samskivert/depot/impl/MySQLBuilder.java

@@ -299,7 +299,7 @@ public class MySQLBuilder
             return "TIME";
         }
         public String getTimestampType (int length) {
-            return "DATETIME";
+            return "TIMESTAMP";
         }
         public String getBlobType (int length) {
             // semi-arbitrarily use VARBINARY() up to 32767