WL#1127: Optimize fatal_error and kill usage

Affects: Server-5.0   —   Status: Un-Assigned   —   Priority: Medium

The idea with this task is to make it faster and easier to detect is
something went wrong.

Currently we have to the following tests in sql_parse.cc:

in handle_one_connection:

    while (!net->error && net->vio != 0 && !thd->killed)
      if (do_command(thd))

This should be replaced with:

    while (!thd->killed)

(In other words, we should set thd->killed both if do_command() would
otherwise return true, which should only happen on 'quit', or when
net->error or net->vio are set.  The later should never be set after
the loop has been entered.

In dispatch_command() we have:

 while (!thd->killed && !thd->is_fatal_error && thd->lex.found_colon)

This should be replaced with:

 while (!thd->killed && thd->lex.found_colon)

In other words, we should also set thd->killed when we set
thd->is_fatal_error.  This will make this and other errors easier to

The above should be relatively easy to do after dimitri's patch where
thd->killed can be used to only kill a running query.
(We probably have to add a new killed state for the case of 'fatal_error',
which should work like KILL_QUERY).