CVectorWriter is useful for overwriting or appending an existing byte vector.
CNetMsgMaker is a shortcut for creating messages on-the-fly which are suitable
for pushing to CConnman.
Adds an `obfuscate` parameter to `CLevelDBWrapper` and makes use of it
for all new chainstate stores built via `CCoinsViewDB`. Also adds an
`Xor` method to `CDataStream`.
Thanks to @sipa @laanwj @pstratem @dexX7 @KyrosKrane @gmaxwell.
