Fragmented Compressed Row Events Aren't Actually Compressed

MDEV-32570 added fragmentation of large row events. When binlog compression is enabled (log_bin_compress=ON), the compression is bypassed, and these binary log events remain as regular row events, when they should be compressed row events.

This is because the compression happens during the Log_event::write() function, whereas row data fragmentation happens before Log_event::write() is called. The Rows_log_event super-class stores the row data buffers to be written to disk. The regular Rows_log_event sub-classes's implementations of Log_event::write() write this data as-is. The compressed sub-classes's implementation of ::write() over-write these buffers with the compressed rows data before writing to disk.

To fragment a large row event, the server fragments the Rows_log_event's row data buffers, to be written by multiple Partial_rows_log_event}}s, and each {{Partial_rows_log_event::write() call writes its portion of the row data buffer to disk. However, this happens before compression ever has a chance to take place, and thereby, an event that should be compressed, never is.

This results in unexpectedly large binlog files and network bandwidth, as the data won't be compressed.