Follow-on cleanup for the transition table patch.

Commit 59702716 added transition table support to PL/pgsql so that
SQL queries in trigger functions could access those transient
tables.  In order to provide the same level of support for PL/perl,
PL/python and PL/tcl, refactor the relevant code into a new
function SPI_register_trigger_data.  Call the new function in the
trigger handler of all four PLs, and document it as a public SPI
function so that authors of out-of-tree PLs can do the same.

Also get rid of a second QueryEnvironment object that was
maintained by PL/pgsql.  That was previously used to deal with
cursors, but the same approach wasn't appropriate for PLs that are
less tangled up with core code.  Instead, have SPI_cursor_open
install the connection's current QueryEnvironment, as already
happens for SPI_execute_plan.

While in the docs, remove the note that transition tables were only
supported in C and PL/pgSQL triggers, and correct some ommissions.

Thomas Munro with some work by Kevin Grittner (mostly docs)

doc/src/sgml/ref/create_trigger.sgml

@@ -8,6 +8,11 @@ PostgreSQL documentation
   <primary>CREATE TRIGGER</primary>
  </indexterm>
 
+ <indexterm>
+  <primary>transition tables</primary>
+  <seealso>ephemeral named relation</seealso>
+ </indexterm>
+
  <refmeta>
   <refentrytitle>CREATE TRIGGER</refentrytitle>
   <manvolnum>7</manvolnum>
@@ -322,11 +327,6 @@ UPDATE OF <replaceable>column_name1</replaceable> [, <replaceable>column_name2</
      <para>
       The (unqualified) name to be used within the trigger for this relation.
      </para>
-     <note>
-      <para>
-       So far only triggers written in C or PL/pgSQL support this.
-      </para>
-     </note>
     </listitem>
    </varlistentry>