createTable method Null safety
Builds a BlockRepository that will use db for persistence.
It calls createTable to make sure the table exists.
Implementation
void createTable() => db.execute('''
CREATE TABLE IF NOT EXISTS $table (
$columnId BLOB PRIMARY KEY NOT NULL,
$columnVersion INTEGER NOT NULL,
$columnPreviousHash BLOB,
$columnTransactionRoot BLOB,
$columnTimestamp INTEGER);
''');