Issue 45 in weed-fs: [Compact issue] Offset overflow

New issue 45 by hieu.hcmus@gmail.com: [Compact issue] Offset overflow
http://code.google.com/p/weed-fs/issues/detail?id=45

You are using uint32(Maximum 4Gb) to store needle offset(Maximum 32Gb)
when compacting.
Currently It is ok if the volume size is < 4gb
Change variable "offset" in ScanVolumeFile function to uint64 to fix the
issue.

go/storage/needle.go

@@ -19,6 +19,9 @@ const (
 	NeedleChecksumSize = 4
 )
 
+/*
+* Needle file size is limited to 4GB for now.
+ */
 type Needle struct {
 	Cookie uint32 `comment:"random number to mitigate brute force lookups"`
 	Id     uint64 `comment:"needle id"`