The problem occurred because the Spider node was incorrectly handling
timestamp values sent to and received from the data nodes.

The problem has been corrected as follows:
- Added logic to set and maintain the UTC time zone on the data nodes.
  To prevent timestamp ambiguity, it is necessary for the data nodes to use
  a time zone such as UTC which does not have daylight savings time.
- Removed the spider_sync_time_zone configuration variable, which did not
  solve the problem and which interfered with the solution.
- Added logic to convert to the UTC time zone all timestamp values sent to
  and received from the data nodes.  This is done for both unique and
  non-unique timestamp columns.  It is done for WHERE clauses, applying to
  SELECT, UPDATE and DELETE statements, and for UPDATE columns.
- Disabled Spider's use of direct update when any of the columns to update is
  a timestamp column.  This is necessary to prevent false duplicate key value
  errors.
- Added a new test spider.timestamp to thoroughly test Spider's handling of
  timestamp values.

Author:
  Jacob Mathew.

Reviewer:
  Kentoku Shiba.

Cherry-Picked:
  Commit 97cc9d34 on branch bb-10.3-MDEV-16246