With innodb_fast_shutdown=3, skip the rollback of connected transactions

Sometimes it would be useful to have a faster clean shutdown where the rollback of current transactions is skipped. These transactions would be rolled back on the background on subsequent server startup.

Let us implement a new mode innodb_fast_shutdown=3 for this. It should be slower than the crash-like (unclean) shutdown with innodb_fast_shutdown=2, because the buffer pool will not be written back to data files.