[Bug] Fix wallet data race

[panleone]

First commit is a small refactor: the global variable nTimeBestReceived is now a private member of the CWallet class, as it was used only there.

Second commit: fix the following data race by making nTimeBestReceived atomic

WARNING: ThreadSanitizer: data race (pid=18270)
  Write of size 8 at 0x7b6800000ed8 by thread T16:
    #0 CWallet::UpdatedBlockTip(CBlockIndex const*, CBlockIndex const*, bool) wallet/wallet.cpp:2094 (pivxd+0x5058c9)
    #1 void std::__invoke_impl<void, void (CValidationInterface::*&)(CBlockIndex const*, CBlockIndex const*, bool), CValidationInterface*&, CBlockIndex const*, CBlockIndex const*, bool>(std::__invoke_memfun_deref, void (CValidationInterface::*&)(CBlockIndex const*, CBlockIndex const*, bool), CValidationInterface*&, CBlockIndex const*&&, CBlockIndex const*&&, bool&&) /usr/include/c++/12/bits/invoke.h:74 (pivxd+0x2ee1c0)
    #2 std::__invoke_result<void (CValidationInterface::*&)(CBlockIndex const*, CBlockIndex const*, bool), CValidationInterface*&, CBlockIndex const*, CBlockIndex const*, bool>::type std::__invoke<void (CValidationInterface::*&)(CBlockIndex const*, CBlockIndex const*, bool), CValidationInterface*&, CBlockIndex const*, CBlockIndex const*, bool>(void (CValidationInterface::*&)(CBlockIndex const*, CBlockIndex const*, bool), CValidationInterface*&, CBlockIndex const*&&, CBlockIndex const*&&, bool&&) /usr/include/c++/12/bits/invoke.h:96 (pivxd+0x2ee25a)
    #3 void std::_Bind<void (CValidationInterface::*(CValidationInterface*, std::_Placeholder<1>, std::_Placeholder<2>, std::_Placeholder<3>))(CBlockIndex const*, CBlockIndex const*, bool)>::__call<void, CBlockIndex const*&&, CBlockIndex const*&&, bool&&, 0ul, 1ul, 2ul, 3ul>(std::tuple<CBlockIndex const*&&, CBlockIndex const*&&, bool&&>&&, std::_Index_tuple<0ul, 1ul, 2ul, 3ul>) /usr/include/c++/12/functional:484 (pivxd+0x2ee25a)
    ...

  Previous read of size 8 at 0x7b6800000ed8 by thread T35 (mutexes: write M134298, write M132880):
    #0 CWallet::ResendWalletTransactions(CConnman*) wallet/wallet.cpp:2065 (pivxd+0x515b25)
    #1 void std::__invoke_impl<void, void (CValidationInterface::*&)(CConnman*), CValidationInterface*&, CConnman*>(std::__invoke_memfun_deref, void (CValidationInterface::*&)(CConnman*), CValidationInterface*&, CConnman*&&) /usr/include/c++/12/bits/invoke.h:74 (pivxd+0x2eef45)
    #2 std::__invoke_result<void (CValidationInterface::*&)(CConnman*), CValidationInterface*&, CConnman*>::type std::__invoke<void (CValidationInterface::*&)(CConnman*), CValidationInterface*&, CConnman*>(void (CValidationInterface::*&)(CConnman*), CValidationInterface*&, CConnman*&&) /usr/include/c++/12/bits/invoke.h:96 (pivxd+0x2eefb9)
    #3 void std::_Bind<void (CValidationInterface::*(CValidationInterface*, std::_Placeholder<1>))(CConnman*)>::__call<void, CConnman*&&, 0ul, 1ul>(std::tuple<CConnman*&&>&&, std::_Index_tuple<0ul, 1ul>) /usr/include/c++/12/functional:484 (pivxd+0x2eefb9)
    ...

[Duddino]

utACK f9a6ebd

[Liquid369]

tACK f9a6ebd

[Fuzzbawls]

ACK f9a6ebd