optim: mark a few classes final #10809

Using gcc's -Wsuggest-final-types and lto, I identified a few easy devirtualization wins:

wallet/wallet.h:651:7: warning: Declaring type 'struct CWallet' final would enable devirtualization of 26 calls [-Wsuggest-final-types]

coins.h:201:7: warning: Declaring type 'struct CCoinsViewCache' final would enable devirtualization of 13 calls [-Wsuggest-final-types]

txdb.h:67:7: warning: Declaring type 'struct CCoinsViewDB' final would enable devirtualization of 5 calls [-Wsuggest-final-types]

zmq/zmqnotificationinterface.h:16:7: warning: Declaring type 'struct CZMQNotificationInterface' final would enable devirtualization of 4 calls [-Wsuggest-final-types]

httpserver.cpp:42:7: warning: Declaring type 'struct HTTPWorkItem' final would enable devirtualization of 2 calls [-Wsuggest-final-types]

test/coins_tests.cpp:74:7: error: cannot derive from ‘final’ base ‘CCoinsViewCache’ in derived type ‘{anonymous}::CCoinsViewCacheTest’ class CCoinsViewCacheTest : public CCoinsViewCache

test/coins_tests.cpp:74:7: error: cannot derive from ‘final’ base ‘CCoinsViewCache’ in derived type ‘{anonymous}::CCoinsViewCacheTest’ class CCoinsViewCacheTest : public CCoinsViewCache

You can fix that by renaming the existing CCoinsViewCache to a non-final CCoinsViewCacheBase, and have CCoinsViewCache become a final subclass of that with no implementation except a constructor. Then CCoinsViewCacheTest can become another subclass of CCoinsViewCacheBase.

@theuni Two more devirtualization opportunities (from closed PR #10847):

• Declaring CCoinsViewErrorCatcher final enables devirtualization of two calls
• Declaring CReserveKey final enables devirtualization of one call

@practicalswift I'll pull in the other two, but I think we should drop the CCoinsViewCache one. The inheritance there is hard enough to follow already.

@theuni Sounds great! :-)

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