Optimize unqualified delete for sharded tables

Before we gained truncate support, people attempted to use delete without a where clause to delete all data from the table.

At some point we'd like to optimize this scenario despite the fact that we have truncate now (since in general semantics of truncate and unqualified delete is different in relation to triggers which we dont have now)

Ref: #1486 (closed) #1737 (closed)