FoundBlock class allows interfaces::Chain::findBlock to return more block
information without having lots of optional output parameters. FoundBlock class
is also used by other chain methods in upcoming commits.

There is mostly no change in behavior. Only exception is
CWallet::RescanFromTime now throwing NonFatalCheckError instead of
std::logic_error.

This is part [1/12] of Core PR17954 : https://github.com/bitcoin/bitcoin/pull/17954/commits/bf30cd4922ea62577d7bf63f5029e8be62665d45