innodb_change_buffering, with the following values:

none - buffer nothing
inserts - buffer inserts (like InnoDB so far)
deletes - buffer delete-marks
changes - buffer inserts and delete-marks
purges - buffer delete-marks and deletes
all - buffer all operations (insert, delete-mark, delete)

The default is 'all'. All values except 'none' and 'inserts' will make
InnoDB+ write new-format records to the insert buffer, even for inserts.

We will implement this variable in the InnoDB Plugin 1.0.3 with the values
'none' and 'inserts' (the default).

This patch also adds a #if 0 TODO snippet for tagging the insert buffer
format in the system tablespace. This is related to
https://svn.innodb.com/innobase/Saving_last_shutdown_state and Issue #81.

rb://79 approved by Heikki Tuuri and Ken Jacobs.