In a multi-batch recovery, we must ensure that INIT_PAGE and
especially the MDEV-15528 FREE_PAGE records will be taken
properly into account.

Writing a FREE_PAGE record gives the server permission to omit
a page write. If recovery insists on applying log to a page
whose page flush has been omitted, then the consistency checks
in the application of high-level redo log records (appending
an undo log record, inserting or deleting an index record)
will likely fail.

mlog_init_t::add(): Return whether the state was changed.

mlog_init_t::will_avoid_read(): Determine whether a page read
will be avoided and whether older log records can be safely
skipped.

recv_sys_t::parse(): Even if store==STORE_NO, process the records
INIT_PAGE and FREE_PAGE. While processing them, we can delete older
redo log records for the page. If store!=STORE_NO, we can directly
skip redo log recods of other types if mlog_init indicates that the
page will be freed or initialized by at a later LSN.

This fix was developed in cooperation with
Thirunarayanan Balathandayuthapani.