Get rid of WALBufMappingLock

Get rid of WALBufMappingLock

Allow multiple backends to initialize WAL buffers concurrently.  This way
`MemSet((char *) NewPage, 0, XLOG_BLCKSZ);` can run in parallel without
taking a single LWLock in exclusive mode.

The new algorithm works as follows:
 * reserve a page for initialization using XLogCtl->InitializeReserved,
 * ensure the page is written out,
 * once the page is initialized, try to advance XLogCtl->InitializedUpTo and
   signal to waiters using XLogCtl->InitializedUpToCondVar condition
   variable,
 * repeat previous steps until we reserve initialization up to the target
   WAL position,
 * wait until concurrent initialization finishes using a
   XLogCtl->InitializedUpToCondVar.

Now, multiple backends can, in parallel, concurrently reserve pages,
initialize them, and advance XLogCtl->InitializedUpTo to point to the latest
initialized page.

Author: Yura Sokolov 
Co-authored-by: Alexander Korotkov 
Reviewed-by: Pavel Borisov 
Reviewed-by: Tomas Vondra 
Tested-by: Michael Paquier