Make handlers and atpds overflows unlikely.

• Allocate 256 handlers more than payload commands for management purposes.
• Increase maximum number of handlers from 8K to 16K by tuning the format.
• Just to be safe limit the number of payload commands to 16K - 256.
• Limit number of target exchanges in mixed mode to the number of atpds.
• If we still somehow get out of atpds -- return BUSY, since we really are.