Code review for ARC patch. Eliminate static variables, improve handling

of VACUUM cases so that VACUUM requests don't affect the ARC state at all,
avoid corner case where BufferSync would uselessly rewrite a buffer that
no longer contains the page that was to be flushed.  Make some minor
other cleanups in and around the bufmgr as well, such as moving PinBuffer
and UnpinBuffer into bufmgr.c where they really belong.