"max-journal-size default" is based on the wrong DB size in 9.16

In the work for #1515 (closed) (max-ixfr-ratio) it became clear that dns_db_getsize() was not returning the correct database size in bytes - it returns the sum of the sizes of all the rdataslabs in the database, which have a great deal of extra overhead space in them. This was corrected in master, but should also be fixed in 9.16 (even if we don't backport max-ixfr-ratio), because it's used to compute the maximum journal size when max-journal-size is set to default.