validation: rename snapshot chainstate dir

This changes the snapshot's leveldb chainstate dir name from
`chainstate_[blockhash]` to `chainstate_snapshot`. This simplifies
later logic that loads snapshot data, and enforces the limitation
of a single snapshot at any given time.

Since we still need to persis the blockhash of the base block, we
write that out to a file (`chainstate_snapshot/base_blockhash`) for
later use during initialization, so that we can reinitialize the
snapshot chainstate.

Co-authored-by: Russell Yanofsky <russ@yanofsky.org>

src/streams.h

@@ -488,12 +488,14 @@ public:
     AutoFile(const AutoFile&) = delete;
     AutoFile& operator=(const AutoFile&) = delete;
 
-    void fclose()
+    int fclose()
     {
+        int retval{0};
         if (file) {
-            ::fclose(file);
+            retval = ::fclose(file);
             file = nullptr;
         }
+        return retval;
     }
 
     /** Get wrapped FILE* with transfer of ownership.