Implement shared memory on Windows

[ncruces]

Port #71 to Windows. Mostly:

• internal/util/alloc.go
• internal/util/mmap.go

Also, vfs/shm.go needs tweaks to the DMS lock.

Reference:
https://devblogs.microsoft.com/oldnewthing/20240201-00/?p=109346

[ncruces]

Virtual memory placeholders look unfeasible for this. We would need to hard code the blocks of memory that could be used for file mappings. It gets rather ugly.

But Windows can likely reuse significant portions of this of #180. It could trade the in memory locks for file locks, map the file in main memory, and support multiple processes sharing the WAL. If so, care needs to be taken that copies will no longer be made under a lock (they may need to be made on the right side of a file lock).