Implement a 10.4 redo log format, which extends the 10.3 format
by introducing the MLOG_MEMSET record.

MLOG_MEMSET: A new redo log record type for filling an area with a byte.

mlog_memset(): Write the MLOG_MEMSET record.

mlog_parse_nbytes(): Handle MLOG_MEMSET as well.

trx_rseg_header_create(): Reduce the redo log volume by making use of
mlog_memset() and the zero-initialization that happens inside page
allocation.

fil_addr_null: Remove.

flst_init(): Create a variant that takes a zero-initialized
buf_block_t* as a parameter, and only writes the FIL_NULL using
mlog_memset().

flst_zero_addr(): A variant of flst_write_addr() that writes
a null address using mlog_memset() for the FIL_NULL.

The following fixes are replacing some use of MLOG_WRITE_STRING
with the more compact MLOG_MEMSET record, or eliminating
redundant redo log writes:

btr_store_big_rec_extern_fields(): Invoke mlog_memset() for
zero-initializing the tail of the ROW_FORMAT=COMPRESSED BLOB page.

trx_sysf_create(), trx_rseg_format_upgrade(): Invoke mlog_memset()
for zero-initializing the page trailer.

fsp_header_init(), trx_rseg_header_create():
Remove redundant zero-initializations.