InnoDB divides the allocation of undo logs into rollback segments.
The DB_ROLL_PTR system column of clustered indexes can address up to
128 rollback segments (TRX_SYS_N_RSEGS). Originally, InnoDB only
created one rollback segment. In MySQL 5.5 or in the InnoDB Plugin
for MySQL 5.1, all 128 rollback segments were created.

MySQL 5.7 hard-codes the rollback segment IDs 1..32 for temporary undo logs.
On upgrade, unless a slow shutdown (innodb_fast_shutdown=0)
was performed on the old server instance, these rollback segments
could be in use by transactions that are in XA PREPARE state or
transactions that were left behind by a server kill followed by a
normal shutdown immediately after restart.

Persistent tables cannot refer to temporary undo logs or vice versa.
Therefore, we should keep two distinct sets of rollback segments:
one for persistent tables and another for temporary tables. In this way,
all 128 rollback segments will be available for both types of tables,
which could improve performance. Also, MariaDB 10.2 will remain more
compatible than MySQL 5.7 with data files from earlier versions of
MySQL or MariaDB.

trx_sys_t::temp_rsegs[TRX_SYS_N_RSEGS]: A new array of temporary
rollback segments. The trx_sys_t::rseg_array[TRX_SYS_N_RSEGS] will
be solely for persistent undo logs.

srv_tmp_undo_logs. Remove. Use the constant TRX_SYS_N_RSEGS.

srv_available_undo_logs: Change the type to ulong.

trx_rseg_get_on_id(): Remove. Instead, let the callers refer to
trx_sys directly.

trx_rseg_create(), trx_sysf_rseg_find_free(): Remove unneeded parameters.
These functions only deal with persistent undo logs.

trx_temp_rseg_create(): New function, to create all temporary rollback
segments at server startup.

trx_rseg_t::is_persistent(): Determine if the rollback segment is for
persistent tables.

trx_sys_is_noredo_rseg_slot(): Remove. The callers must know based on
context (such as table handle) whether the DB_ROLL_PTR is referring to
a persistent undo log.

trx_sys_create_rsegs(): Remove all parameters, which were always passed
as global variables. Instead, modify the global variables directly.

enum trx_rseg_type_t: Remove.

trx_t::get_temp_rseg(): A method to ensure that a temporary
rollback segment has been assigned for the transaction.

trx_t::assign_temp_rseg(): Replaces trx_assign_rseg().

trx_purge_free_segment(), trx_purge_truncate_rseg_history():
Remove the redundant variable noredo=false.
Temporary undo logs are discarded immediately at transaction commit
or rollback, not lazily by purge.

trx_purge_mark_undo_for_truncate(): Remove references to the
temporary rollback segments.

trx_purge_mark_undo_for_truncate(): Remove a check for temporary
rollback segments. Only the dedicated persistent undo log tablespaces
can be truncated.

trx_undo_get_undo_rec_low(), trx_undo_get_undo_rec(): Add the
parameter is_temp.

trx_rseg_mem_restore(): Split from trx_rseg_mem_create().
Initialize the undo log and the rollback segment from the file
data structures.

trx_sysf_get_n_rseg_slots(): Renamed from
trx_sysf_used_slots_for_redo_rseg(). Count the persistent
rollback segment headers that have been initialized.

trx_sys_close(): Also free trx_sys->temp_rsegs[].

get_next_redo_rseg(): Merged to trx_assign_rseg_low().

trx_assign_rseg_low(): Remove the parameters and access the
global variables directly. Revert to simple round-robin, now that
the whole trx_sys->rseg_array[] is for persistent undo log again.

get_next_noredo_rseg(): Moved to trx_t::assign_temp_rseg().

srv_undo_tablespaces_init(): Remove some parameters and use the
global variables directly. Clarify some error messages.

Adjust the test innodb.log_file. Apparently, before these changes,
InnoDB somehow ignored missing dedicated undo tablespace files that
are pointed by the TRX_SYS header page, possibly losing part of
essential transaction system state.