Bug #45949 Assertion `!tables->table' in open_tables() on 
           ALTER + INSERT DELAYED

The assertion was caused by improperly closing tables when 
INSERT DELAYED needed to reopen tables. This patch replaces
the call to close_thread_tables with close_tables_for_reopen
which fixes the problem.

The only way I was able to trigger the reopen code path and 
thus the assertion, was if ALTER TABLE killed the delayed
insert thread and the delayed insert thread was able to enter
the reopen code path before it noticed that thd->killed had
been set. Note that in these cases reopen will always fail 
since open_table() will check thd->killed and return. This patch
therefore adds two more thd->killed checks to minimize the 
chance of entering the reopen code path without hope for success.

The patch also changes it so that if the delayed insert is killed
using KILL_CONNECTION, the error message that is copied to the
connection thread is ER_QUERY_INTERRUPTED rather than 
ER_SERVER_SHUTDOWN. This means that if INSERT DELAYED fails, 
the user will now see "Query execution was interrupted" rather
than the misleading "Server shutdown in progress".

No test case is supplied. This is for two reasons:
1) Unable to reproduce the error without having the delayed insert
thread in a killed state which means that reopen is futile and
was not supposed to be attempted.
2) Difficulty of using sync points in other threads than 
the connection thread.
The patch has been successfully tested with the RQG and the grammar
supplied in the bug description.