This fixes memory leaks in tests that cause InnoDB startup to fail.

buf_pool_free_instance(): Also free buf_pool->flush_rbt, which would
normally be freed when crash recovery finishes.

fil_node_close_file(), fil_space_free_low(), fil_close_all_files():
Relax some debug assertions to tolerate !srv_was_started.

innodb_shutdown(): Renamed from innobase_shutdown_for_mysql().
Changed the return type to void. Do not assume that all subsystems
were started.

que_init(), que_close(): Remove (empty functions).

srv_init(), srv_general_init(): Remove as global functions.

srv_free(): Allow srv_sys=NULL.

srv_get_active_thread_type(): Only return SRV_PURGE if purge really
is running.

srv_shutdown_all_bg_threads(): Do not reset srv_start_state. It will
be needed by innodb_shutdown().

innobase_start_or_create_for_mysql(): Always call srv_boot() so that
innodb_shutdown() can assume that it was called. Make more subsystems
dependent on SRV_START_STATE_STAT.

srv_shutdown_bg_undo_sources(): Require SRV_START_STATE_STAT.

trx_sys_close(): Do not assume purge_sys!=NULL. Do not call
buf_dblwr_free(), because the doublewrite buffer can exist while
the transaction system does not.

logs_empty_and_mark_files_at_shutdown(): Do a faster shutdown if
!srv_was_started.

recv_sys_close(): Invoke dblwr.pages.clear() which would normally
be invoked by buf_dblwr_process().

recv_recovery_from_checkpoint_start(): Always release log_sys->mutex.

row_mysql_close(): Allow the subsystem not to exist.