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.

mysql-test/t/function_defaults_notembedded.test

@@ -0,0 +1,18 @@
+--echo #
+--echo # Test of function defaults for non-embedded server.
+--echo #
+
+--source include/not_embedded.inc
+--source include/have_debug_sync.inc
+
+--echo #
+--echo # Function defaults run 1. No microsecond precision.
+--echo #
+let $timestamp=TIMESTAMP;
+--source include/function_defaults_notembedded.inc
+
+--echo #
+--echo # Function defaults run 2. Six digits scale on seconds precision.
+--echo #
+let $timestamp=TIMESTAMP(6);
+--source include/function_defaults_notembedded.inc