Fix checkpointer crash in EXEC_BACKEND builds.

Fix checkpointer crash in EXEC_BACKEND builds.

Nothing in the checkpointer calls InitXLOGAccess(), so WALInsertLocks
never got initialized there.  Without EXEC_BACKEND, it works anyway
because the correct value is inherited from the postmaster, but
with EXEC_BACKEND we've got a problem.  The problem appears to have
been introduced by commit 68a2e52bbaf98f136a96b3a0d734ca52ca440a95.

To fix, move the relevant initialization steps from InitXLOGAccess()
to XLOGShmemInit(), making this more parallel to what we do
elsewhere.

Amit Kapila