matched_rec::rec_buf[], matched_rec::bufp: Remove.

matched_rec::block: Make this a pointer to something that
is allocated by buf_block_alloc(). In this way, the only
case where buf_block_t is constructed outside buf_pool
is ALTER TABLE...IMPORT TABLESPACE.

rtr_info::heap: Remove. This was only used for allocating matched_rec,
which now is smaller.

mtr_t::memmove(): Simplify some code to avoid GCC 9.4.0 -Wconversion
in the 10.6 branch as a result of these changes.

Reviewed by: Debarun Banerjee