WL#1657: Better Dynamic Record Format with MyISAM

Re-design the layout of dynamic records in MyISAM so that we have less
fragmentationin the data file and can do minor in-place record extension without
record moving. This could perhaps be accomplished by a grid of N bytes on the
file, so that every record segment occupies a multiple of this size and in the
average has half of the grid room for in-place extension.

Monty's suggestion, added by Trudy Pelzer 2005-10-03
Store fixed-size data in each row before dynamic-size data.
(Note that this is common with other DBMSs.)