Merge #18241: wallet/refactor: refer to CWallet immutably when possible

79facb11e9 wallet: use constant CWallets in rpcwallet.cpp (Karl-Johan Alm)
d9b0ebc1da wallet: make ReserveDestination pwallet ivar const (Karl-Johan Alm)
57c569e4d9 wallet: make BackupWallet() const (Karl-Johan Alm)
df3a818d2a wallet: make getters const (Karl-Johan Alm)
227b9dd2d6 wallet/spkm: make GetOldestKeyPoolTime() const (Karl-Johan Alm)
22d329ad0e wallet: use constant CWallets in rpcdump.cpp (Karl-Johan Alm)
7b3587b29d wallet/db: make IsDummy() const (Karl-Johan Alm)
d366795d18 wallet/db: make Backup() const (Karl-Johan Alm)
8cd0b86340 wallet: make CanGetAddresses() const (Karl-Johan Alm)
037fa770eb wallet: make KeypoolCountExternalKeys() const (Karl-Johan Alm)
ddc93557ad wallet: make CanGenerateKeys() const (Karl-Johan Alm)
dc2d0650fd make BlockUntilSyncedToCurrentChain() const (Karl-Johan Alm)

Pull request description:

  A lot of places refer to `CWallet*`'s as `CWallet * const`, which translates to *"an immutable pointer to a mutable `CWallet` instance"*; this is

  1. often not what the author meant, especially as a lot of these places do not at all modify the wallet object, and
  2. confusing, as it tends to suggest that this is a proper way to refer to a constant `CWallet` instance.

  This PR changes references to wallets to `const CWallet* const` whenever immutability is expected. This should result in no behavioral changes at all, and improved compile-time error checking.

  Note from irc:

  > <sipa> sounds good to me; this is the sort of change that as long as it compiles, the behavior shouldn't change
  > <sipa> though in general it may lead to introducing automatic copying of objects sometimes (e.g. trying to std::move a const object will work, but generally result in a copy rather than an efficient move)
  > <sipa> CWallet objects aren't copied or moved though

ACKs for top commit:
  laanwj:
    ACK 79facb11e9
  Empact:
    ACK 79facb11e9
  promag:
    ACK 79facb11e9.
  fjahr:
    ACK 79facb11e9

Tree-SHA512: 80a80c1a52f0f788d0ccb268b53bc0f46c796643a3c5a22b55bbbde4ffa6c7e347784e5e53b1e488a3b4e14399e31d5be9417ad5b6319c74a462609e9b1a98e8

src/wallet/db.cpp

@@ -850,7 +850,7 @@ bool BerkeleyDatabase::Rewrite(const char* pszSkip)
     return BerkeleyBatch::Rewrite(*this, pszSkip);
 }
 
-bool BerkeleyDatabase::Backup(const std::string& strDest)
+bool BerkeleyDatabase::Backup(const std::string& strDest) const
 {
     if (IsDummy()) {
         return false;