Added one_small_row, one_big_row, many_rows, stmt_params benchmarks against libmysqlclient and libmariadb
Added a CI build to compile and run benchmarks
Added a Python script to run the benchmarks
Refactored the connection_pool benchmark to be use data independent from examples
close#458
Added tutorials 2, 3 and 4
Changed most of the discussion to use any_connection
Added examples on disabling TLS, multi-function operations, multi-queries and transactions
All examples now use any_connection
Automate parts of example qbk generation
Partially addresses #365 and #366
Introduced BOOST_MYSQL_DISABLED_SERVER_FEATURES env var
Removed BOOST_MYSQL_NO_UNIX_SOCKET_TESTS env var
JSON tests in database_types are now actually run
close#29
