Avoid "could not reattach" by providing space for concurrent allocation.

We've long had reports of intermittent "could not reattach to shared
memory" errors on Windows.  Buildfarm member dory fails that way when
PGSharedMemoryReAttach() execution overlaps with creation of a thread
for the process's "default thread pool".  Fix that by providing a second
region to receive asynchronous allocations that would otherwise intrude
into UsedShmemSegAddr.  In pgwin32_ReserveSharedMemoryRegion(), stop
trying to free reservations landing at incorrect addresses; the caller's
next step has been to terminate the affected process.  Back-patch to 9.4
(all supported versions).

Reviewed by Tom Lane.  He also did much of the prerequisite research;
see commit bcbf2346d6.

Discussion: https://postgr.es/m/20190402135442.GA1173872@rfd.leadboat.com

src/include/storage/pg_shmem.h

@@ -65,6 +65,7 @@ typedef enum
 extern unsigned long UsedShmemSegID;
 #else
 extern HANDLE UsedShmemSegID;
+extern void *ShmemProtectiveRegion;
 #endif
 extern void *UsedShmemSegAddr;