More code for issue #34

Introduced table default_hostgroups and runtime_default_hostgroups
Added functions to read and write these tables
Added functions to read and write structure gloDefHG
Added command FLUSH DEFAULT HOSTGROUPS
Added Command DUMP RUNTIME DEFAULT HOSTGROUPS
Table default_hostgroups is read at startup
Table default_hostgroups is synced with CONFIG SYNC TO DISK
sess->default_hostgroup is reset when COM_INIT_DB
sess->default_hostgroup is updated when reset or new version is available