Fix assert failure at end of recovery, broken by XLogInsert scaling patch.

 Fix assert failure at end of recovery, broken by XLogInsert scaling patch.
 
 Initialization of the first XLOG buffer at end-of-recovery was broken for
 the case that the last read WAL record ended at a page boundary. Instead of
 trying to copy the last full xlog page to the buffer cache in that case,
 just set shared state so that the next page is initialized when the first
 WAL record after startup is inserted. (that's what we did in earlier
 version, too)
 
 To make the shared state required for that case less surprising, replace the
 XLogCtl->curridx variable, which was the index of the latest initialized
 buffer, with an XLogRecPtr of how far the buffers have been initialized.
 That also allows us to get rid of the XLogRecEndPtrToBufIdx macro.
 
 While we're at it, make a similar change for XLogCtl->Write.curridx, getting
 rid of that variable and calculating the next buffer to write from
 XLogCtl->LogwrtResult instead.