Improve DECIMAL support to store and compare packed decimals

[greenlion]

This is an on-disk format change, so it should be part of beta 2 so that tables do not need to be repaired/reconstructed in beta3.

[greenlion]

In beta2 WARP continues to store decimal as string, and it pushes decimal comparisons to the MySQL layer. Doing decimal comparisons requires Fastbit modification of both bitmap indexing and column comparisons and is too complex for beta 2. Might be pushed out beyond beta 3.