minor: added comment about per thread connections

src/main/scala/lc/database/statements.scala

@@ -110,6 +110,13 @@ class Statements(dbConn: DBConn, maxAttempts: Int) {
 }
 
 object Statements {
+  /* Note: h2 documentation recommends using a separate DB connection per thread
+     But in practice, as of version 1.4.200, multiple connections occassionally shows error on the console of the form
+     ```
+     org.h2.jdbc.JdbcSQLNonTransientException: General error: "java.lang.NullPointerException"; SQL statement:
+     SELECT image FROM challenge c, mapId m WHERE c.token=m.token AND m.uuid = ? [50000-200]
+     ```
+     */
   private val dbConn: DBConn = new DBConn()
   private val maxAttempts = 10
   val tlStmts: ThreadLocal[Statements] = ThreadLocal.withInitial(() => new Statements(dbConn, maxAttempts))