Some rework to try to reduce duplicated code

In Admin:
- replaced a lot of flush_[...]__from_memory_to_disk() and flush_[...]__from_disk_to_memory() with a generic flush_GENERIC__from_to()
- several commands like "LOAD MYSQL SERVERS FROM DISK" and similar are automatically generated for various modules (not all) and saved/map in a map load_save_disk_commands
- FlushCommandWrapper() is able to map the various LOAD/SAVE from/to DISK into commands to run on SQLite

This commit adds some minimal testing for mysql_hostgroup_attributes .
It helped detecting some bugs.

Fixes:
* minor memory leak when running CHECKSUM command
* minor memory leak in load_mysql_servers_to_runtime()
* not initialized json in MySQL_Event (valgrind doesn't like it)
* not initialized variable MySQL_Thread::shutdown in the constructor

If mysql_hostgroup_attributes.multiplex:
- 0 : multiplexing is always disabled
- 1 : multiplexing is enabled for the hostgroup, but other criteria can disable multiplexing (transaction, temp tables, query rules, etc)

Implementation of mysql_hostgroup_attributes.free_connections_pct and mysql_hostgroup_attributes.connection_warming

If mysql_hostgroup_attributes is configured:
- mysql_hostgroup_attributes.free_connections_pct has higher priority than global variable mysql-free_connections_pct
- mysql_hostgroup_attributes.connection_warming has higher priority than global variable mysql-connection_warming

Implementation of mysql_hostgroup_attributes.throttle_connections_per_sec

If mysql_hostgroup_attributes is configured,
mysql_hostgroup_attributes.throttle_connections_per_sec has higher
priority than global variable mysql-throttle_connections_per_sec_to_hostgroup