In mariadb-backup --backup, we only have to invoke the undo_space_trunc
and log_file_op callbacks as well as validate the mini-transaction
checksums. There is absolutely no need to access recv_sys.pages or
recv_spaces. This is what the new mode recv_sys_t::store::BACKUP will do.

In the skip_the_rest: loop, the minimum that needs to be done is to
process all FILE_ records until the end of the log is reached.
Additionally, in case we invoked file_name_t::add_freed_page() for a
FREE_PAGE record before switching to the skip_the_rest: loop, we must
invoke file_name_t::remove_freed_page() for any INIT_PAGE record.
Any other records that we encounter during this parsing can be ignored;
they will eventually be processed on a subsequent call to recv_scan_log()
with store=true.

This was measured to reduce the CPU time between the messages
"InnoDB: Multi-batch recovery needed at LSN" and
"InnoDB: End of log at LSN"
by some 20%.

recv_sys_t::store: A ternary enumeration that specifies how records
should be stored: NO, BACKUP, or YES.

recv_sys_t::parse(), recv_sys_t::parse_mtr(), recv_sys_t::parse_pmem():
Replace template<bool store> with template<store storing>.

store_freed_or_init_rec(): Simplify some logic. We can look up also
the system tablespace.