MDEV-452 Add full support for auto-initialized/updated timestamp and datetime

Generalized support for auto-updated and/or auto-initialized timestamp
and datetime columns. This patch is a reimplementation of MySQL's
"WL#5874: CURRENT_TIMESTAMP as DEFAULT for DATETIME columns". In order to
ease future merges, this implementation reused few function and variable
names from MySQL's patch, however the implementation is quite different.

TODO:
The only unresolved problem in this patch is the semantics of LOAD DATA for
TIMESTAMP and DATETIME columns in the cases when there are missing or NULL
columns. I couldn't fully comprehend the logic behind MySQL's behavior and
its relationship with their own documentation, so I left the results to be
more consistent with all other LOAD cases.

The problematic test cases can be seen by running the test file function_defaults,
and observing the test case differences. Those were left on purpose for discussion.

1 files changed, 2 insertions, 2 deletions

diff --git a/mysql-test/t/create.test b/mysql-test/t/create.test
index d80127df860..258213620ce 100644
--- a/mysql-test/t/create.test
+++ b/mysql-test/t/create.test
@@ -55,10 +55,10 @@ create table a (`aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
 #
 # Some wrong defaults, so these creates should fail too (Bug #5902)
 #
---error 1067
 create table t1 (a datetime default now());
---error 1294
+drop table t1;
 create table t1 (a datetime on update now());
+drop table t1;
 --error 1067
 create table t1 (a int default 100 auto_increment);
 --error 1067