Fix shadow wal corruption on stalled validation

This commit fixes a timing bug that occurs in a specific scenario
where the shadow wal sync stalls because of an s3 validation and
the catch up write to the shadow wal is large enough to allow a
window between WAL reads and the final copy.

The file copy has been replaced by direct writes of the frame
buffer to the shadow to ensure that every validated byte is exactly
what is being written to the shadow wal. The one downside to this
change is that the frame buffer will grow with the transaction
size so it will use additional heap. This can be replaced by a
spill-to-disk implementation but this should work well in the
short term.

litestream.go

@@ -95,9 +95,9 @@ type Pos struct {
 // String returns a string representation.
 func (p Pos) String() string {
 	if p.IsZero() {
-		return "<>"
+		return ""
 	}
-	return fmt.Sprintf("<%s,%08x,%d>", p.Generation, p.Index, p.Offset)
+	return fmt.Sprintf("%s/%08x:%d", p.Generation, p.Index, p.Offset)
 }
 
 // IsZero returns true if p is the zero value.