Change the way that LWLocks for extensions are allocated.

The previous RequestAddinLWLocks() method had several disadvantages.
First, the locks would be in the main tranche; we've recently decided
that it's useful for LWLocks used for separate purposes to have
separate tranche IDs.  Second, there wasn't any correlation between
what code called RequestAddinLWLocks() and what code called
LWLockAssign(); when multiple modules are in use, it could become
quite difficult to troubleshoot problems where LWLockAssign() ran out
of locks.  To fix, create a concept of named LWLock tranches which
can be used either by extension or by core code.

Amit Kapila and Robert Haas

doc/src/sgml/xfunc.sgml

@@ -3335,9 +3335,12 @@ void RequestAddinShmemSpace(int size)
     <para>
      LWLocks are reserved by calling:
 <programlisting>
-void RequestAddinLWLocks(int n)
+void RequestNamedLWLockTranche(const char *tranche_name, int num_lwlocks)
 </programlisting>
-     from <function>_PG_init</>.
+     from <function>_PG_init</>.  This will ensure that an array of
+     <literal>num_lwlocks</> LWLocks is available under the name
+     <literal>tranche_name</>.  Use <function>GetNamedLWLockTranche</>
+     to get a pointer to this array.
     </para>
     <para>
      To avoid possible race-conditions, each backend should use the LWLock
@@ -3356,7 +3359,7 @@ if (!ptr)
         {
                 initialize contents of shmem area;
                 acquire any requested LWLocks using:
-                ptr->mylockid = LWLockAssign();
+                ptr->locks = GetNamedLWLockTranche("my tranche name");
         }
         LWLockRelease(AddinShmemInitLock);
 }