Silence the UNIV_SYNC_DEBUG assertion failure while upgrading old data files
to multiple rollback segments during server startup. Because the upgrade
takes place while InnoDB is running a single thread, we can safely ignore the
latching order checks without fearing deadlocks.
      
innobase_start_or_create_for_mysql(): Set srv_is_being_started = FALSE
only after trx_sys_create_rsegs() has completed.
      
sync_thread_add_level(): If srv_is_being_started, ignore latching order
violations for SYNC_TRX_SYS_HEADER and SYNC_IBUF_BITMAP.

Create all the non-IO threads after creating the extra rollback segments.

Patch originally from Marko with some additions by Sunny.