For undo log truncation, commit 055a3334
repurposed the MLOG_FILE_CREATE2 record with a nonzero page size
to indicate that an undo tablespace will be shrunk in size.
In commit 7ae21b18 the
MLOG_FILE_CREATE2 record was replaced by a FILE_CREATE record.

Now that the redo log encoding was changed, there is no actual need
to write a file name in the log record; it suffices to write the
page identifier of the first page that is not part of the file.

This TRIM_PAGES record could allow us to shrink any data files in the
future. For now, it will be limited to undo tablespaces.

mtr_t::log_file_op(): Remove the parameter first_page_no, because
it would always be 0 for file operations.

mtr_t::trim_pages(): Replaces fil_truncate_log().

mtr_t::log_write(): Avoid same_page encoding if !bpage&&!m_last.

fil_op_replay_rename(): Remove the constant parameter first_page_no=0.