fix bug 1167451 - Fail drop sequence cascade if sequence is used.
Merge trunk, resolve conflicts
move the sequence in-use check to BasicDDLFunctions
merge tjoneslo: Fix bug 1167451 - Have the SequenceDDL process check if the sequence in question is begin used as a identity column for a table and reject the drop with an error message.

https://code.launchpad.net/~tjoneslo/akiban-server/fix-bug-1167541/+merge/159496