Currently MySQL only permits a literal value as column default. This WL
is removing this limitation by also allowing expressions to generate
values as DEFAULT. With these changes the following syntax for
defining/altering a column will be allowed:
<column name> <column data type> DEFAULT (expression)

This worklog reuses some of the infrastructure around Generated Columns
and expands the range of functions that can be used as default
expressions by also allowing the following non-deterministic functions:
uuid, rand, statement_digest, statement_digest_text, curdate,
current_date, curtime, current_time, current_timestamp, localtime,
localtimestamp, now, sysdate, unix_timestamp, utc_date, utc_time,
utc_timestamp, connection_id, database.
Also all the deterministic functions that were previously allowed for
Generated Columns are also allowed as default expressions.

Change-Id: Ieeb72efd87ae3e98349a257c40061bcf2d3ddb22