Closing one of multiple db objects for the same db file causes failure

[brodycj]

While it is possible to support db connection objects to the same db file, closing one of the db connection objects quietly breaks the other db connection.

UPDATED REPRODUCTION SCENARIO: The following reproduction scenario fails with a timeout:

        if (!isWebSql) test(suiteName + ' close one of multiple db handle', function () {
          var dbname = 'multi-handle-db-close-one.db';
          var dbw = openDatabase(dbname, "1.0", "Demo", DEFAULT_SIZE);
          var dbr = openDatabase(dbname, "1.0", "Demo", DEFAULT_SIZE);

          stop(1);

          dbw.transaction(function (tx) {
            tx.executeSql('DROP TABLE IF EXISTS tt');
            tx.executeSql('CREATE TABLE IF NOT EXISTS tt (test_data)');
            tx.executeSql('INSERT INTO tt VALUES (?)', ['My-test-data']);
          }, function(error) {
            console.log("ERROR: " + error.message);
            ok(false, error.message);
            start(1);
          }, function() {
            dbw.close(function () {
              dbr.readTransaction(function (tx) {
                tx.executeSql('SELECT test_data from tt', [], function (tx, result) {
                  equal(result.rows.item(0).test_data, 'My-test-data', 'read data from reader handle');
                  start(1);
                });
              }, function(error) {
                console.log("ERROR: " + error.message);
                ok(false, error.message);
                start(1);
              });
            }, function (error) {
              ok(false, 'close error callback not to be called after database is closed');
              start(1);
            });
          });
        });

This issue was discovered by consideration of some CoffeeScript/Javascript restructuring and possible redesign to support solution to #184.

[brodycj]

IMPORTANT NOTE (implementation): if a db handle object has been closed, it should abort all attempts to execute SQL statements and transactions, and should reject any future attempt to close with an error callback (as already tested in commit 2e31d46).

[brodycj]

Fixed in documentation: using the close method will close the database on the native side and invalidate all handles on the JavaScript side. For reference: https://www.infoq.com/articles/0-bugs-policy

A FUTURE TODO is to add a dispose method that will simply invalidate a single database handle object. The database would be closed on the native side once all handles on the JavaScript side are invalidated.