Changes for v1.6.0

• Refactored the parsing of arrays for engines that don't support arrays (e.g., SQLite & Firebird). A new method in DBIEngine, _parse_array, replaces inline uses of split. This allows the tests to more consistently compare arrays as arrays, though search_events must now always parse tags, requires, and conflicts.
• Added support for ClickHouse. It relies on the clickhouse (or clickhouse-client) CLI and ODBC driver. Like MySQL, it uses a database for the registry schema, where the tables use the MergeTree engine. It supports client certificate authentication and the ClickHouse client configuration file format.