This worklog implements new behavior for TIMESTAMP columns,
defined by CREATE TABLE, ALTER TABLE and CREATE SELECT commands.
This is in effort to make TIMESTAMP columns behavior to be
more closer to SQL standard. The change in behavior is as
described below,

Current behavior:
=================
  1) Unlike all the other types, TIMESTAMP columns which are not
     explicitly specified as NULLable automatically get NOT NULL
     as attribute.
  2) If the first TIMESTAMP column in table is not specified as
     NULLable and doesn't have explicit DEFAULT or ON UPDATE
     value specified it automatically gets DEFAULT NOW()
     ON UPDATE NOW() as attributes.
  3) All other TIMESTAMP columns which are not NULLable and
     don't have explicit default specified get '0' as default
     value and treated as having explicit default value after
     that (i.e. if you don't provide explicit value for such
     a column when inserting into table no warning or error
     is emitted).

New behavior:
=============
  1) TIMESTAMP columns which are not explicitly specified as
     NOT NULL become NULLable.
  2) No TIMESTAMP columns get DEFAULT NOW() or ON UPDATE NOW()
     attributes automatically, without them being explicitly
     specified.
  3) Non-NULLable TIMESTAMP columns without explicit default
     value treated as having no default value. I.e. warning
     or error is emitted (depends on sql_mode) if we insert
     a row without providing and explicit value for such a
     column. In case when warning is emitted and not an
     error such a column will still get '0' if no explicit
     value was specified for it.

A new command line option '--explicit_defaults_for_timestamp'
is introduced for MySQL server to enable new behavior. The old
behavior is kept as default, so that the existing applications
assuming old semantics work without any changes.

Replication slave applier thread is extended with new interface,
such that the slave applies the logs with old behavior, if the
logs are generated by master with older version than slave.

MySQL system table definitions at scripts/* were updated,
such that they work fine with old and new behavior with
any change. MTR tests which use TIMESTAMP columns are also
updated, such that the test works both in old and new
behavior. This should help easy transition to new behavior
going forward.

This worklog addresses Bug#11762529 and Bug#13344629.