local HASHTAB for currently used session variables and low level access functions

Session variables are stored in session memory in a dedicated hash table.
They are set by the LET command and read by the SELECT command.
The access rights should be checked.

Hash entries related to dropped session variables are not released. The memory
cleaning is implemented in memory-cleaning-after-DROP-VARIABLE patch.

Author: [email protected]

doc/src/sgml/glossary.sgml

@@ -1717,8 +1717,9 @@
     <para>
      A persistent database object that holds a value in session memory.  This
      value is private to each session and is released when the session ends.
-     Read or write access to session variables is controlled by privileges,
-     similar to other database objects.
+     The default value of the session variable is null.  Read or write access
+     to session variables is controlled by privileges, similar to other database
+     objects.
     </para>
     <para>
      For more information, see <xref linkend="ddl-session-variables"/>.