InnoDB startup was discovering undo tablespaces in a dirty way.
It was reading a possibly stale copy of the TRX_SYS page before
processing any redo log records.

srv_start(): Do not call buf_pool_invalidate(). Invoke
trx_rseg_get_n_undo_tablespaces() after the recovery has been initiated.

recv_recovery_from_checkpoint_start(): Assert that the buffer pool is
empty. This used to be guaranteed by the buf_pool_invalidate() call.

trx_rseg_get_n_undo_tablespaces(): Move to the calling compilation unit,
and reimplement in a simpler way.

srv_undo_tablespace_create(): Remove the constant parameter
size=SRV_UNDO_TABLESPACE_SIZE_IN_PAGES.

srv_undo_tablespace_open(): Reimplement in a cleaner way, with
more robust error handling.

srv_all_undo_tablespaces_open(): Split from srv_undo_tablespaces_init().

srv_undo_tablespaces_init(): Read all "undo001","undo002" tablespace
files directly, without consulting the TRX_SYS page via calling
trx_rseg_get_n_undo_tablespaces().

This is joint work with Thirunarayanan Balathandayuthapani.