Add full support for DEFAULT

• Support for functions: DEFAULT (1+1)
• Referring to previous fields in the table
• Support for DEFAULT for blobs

In addition this task will lift the current restrictions for virtual columns:

• Non stored virtual columns can have almost any expression (including constant expression)
• No 300 character limit. (Limit is 64K for all stored expressions + column definitions)

Example:

create table t1 (a bigint default uuid_short(), b blob DEFAULT "look at me!" NOT NULL);