Ever since MDEV-5800 enabled indexed virtual columns for InnoDB,
the InnoDB shutdown relied on close_connections() that would set
thd->killed for the InnoDB purge threads. Alas, the embedded server
shutdown is not invoking close_connections(), and thus InnoDB purge
threads fail to initiate shutdown, causing a hang.

innodb_inited: Remove. Use srv_was_started instead.

innobase_fast_shutdown: Remove. Use srv_fast_shutdown instead.

srv_running: Renamed from thd_destructor_myvar, and made global.
The value NULL means that shutdown was requested or the purge threads
should not be running because of innodb_read_only_mode=1.

innobase_init(): Set srv_was_started after ensuring that srv_running
was initialized. (In innodb_read_only mode, the purge threads are not
started and we do not care if srv_running==NULL.)

innobase_start_or_create_for_mysql(): Do not set srv_was_started.
Let it be set by the only caller innobase_init().

srv_purge_should_exit(): Check also srv_was_started and srv_running
when evaluating thd->killed.