rawdb: Database Version must be unsigned integer

[moonfruit]

rawdb write database version as int. It is wrong because of rlp can only handle unsigned integer.
line 40 in accessors_metadata.go actually produced an error, but it ignored. This bug makes database version checking not work.

go-ethereum/core/rawdb/accessors_metadata.go

Line 40 in f850123

enc, _ := rlp.EncodeToBytes(version)