trx_sys_t::undo_log_nonempty: Set to true if there are undo logs
to rollback and purge.

The algorithm for re-creating the undo tablespace when
trx_sys_t::undo_log_nonempty is disabled:

1) trx_sys_t::reset_page(): Reset the TRX_SYS page and assign all
rollback segment slots from 1..127 to FIL_NULL

2) Free the rollback segment header page of system tablespace
for the slots 1..127

3) Update the binlog and WSREP information in system tablespace
rollback segment header
Step (1), (2) and Step (3) should happen atomically within a
single mini-transaction.

4) srv_undo_delete_old_tablespaces(): Delete the old undo tablespaces
present in the undo log directory

5) Make checkpoint to get rid of old undo log tablespaces redo logs

6) Assign new start space id for the undo log tablespaces

7) Re-create the specified undo log tablespaces. InnoDB uses same
mtr for this one and step (6)

8) Make checkpoint again, so that server or mariabackup
can read the undo log tablespace page0 before applying
the redo logs

srv_undo_tablespaces_reinit(): Recreate the undo log tablespaces.
It does reset trx_sys page, delete the old undo tablespaces,
update the binlog offset, write set replication checkpoint
in system rollback segment page

trx_rseg_update_binlog_offset(): Added 2 new parameters to pass
binlog file name and binlog offset

trx_rseg_array_init(): Return error if the rollback segment
slot points to non-existent tablespace

srv_undo_tablespaces_init(): Added new parameter mtr
to initialize all undo tablespaces

trx_assign_rseg_low(): Allow the transaction to use the rollback
segment slots(1..127) even if InnoDB failed to change to the
requested innodb_undo_tablespaces=0

srv_start(): Override the user specified value of
innodb_undo_tablespaces variable with already existing actual
undo tablespaces

wf_incremental_process(): Detects whether TRX_SYS page has been
modified since last backup. If it is then incremental backup
fails and throws the information about taking full backup again

xb_assign_undo_space_start(): Removed the function. Because
undo001 has first undo space id value in page0

Added test case to test the scenario during startup and mariabackup
incremental process too.

Reviewed-by : Marko Mäkelä
Tested-by : Matthias Leich