nFileVersion is not the actual file version and is not used except
in one place. So it is removed from CWalletScanState and changed so
that it is just read at the place it is needed. Furthermore, the
"version" record now only indicates the version of the highest
versioned client that has opened a wallet file so the variable
name is changed accordingly
https://github.com/bitcoin/bitcoin/pull/15588/commits/c88e87c3b2be3f97b712107e04285d06dfef3878
Partial backport of Core PR15588