Merge #8694: Basic multiwallet support

Merge #8694: Basic multiwallet support

c237bd7 wallet: Update formatting (Luke Dashjr)
9cbe8c8 wallet: Forbid -salvagewallet, -zapwallettxes, and -upgradewallet with multiple wallets (Luke Dashjr)
a2a5f3f wallet: Base backup filenames on original wallet filename (Luke Dashjr)
b823a4c wallet: Include actual backup filename in recovery warning message (Luke Dashjr)
84dcb45 Bugfix: wallet: Fix warningStr, errorStr argument order (Luke Dashjr)
008c360 Wallet: Move multiwallet sanity checks to CWallet::Verify, and do other checks on all wallets (Luke Dashjr)
0f08575 Wallet: Support loading multiple wallets if -wallet used more than once (Luke Dashjr)
b124cf0 Wallet: Replace pwalletMain with a vector of wallet pointers (Luke Dashjr)
19b3648 CWalletDB: Store the update counter per wallet (Luke Dashjr)
74e8738 Bugfix: ForceSetArg should replace entr(ies) in mapMultiArgs, not append (Luke Dashjr)
23fb9ad wallet: Move nAccountingEntryNumber from static/global to CWallet (Luke Dashjr)
9d15d55 Bugfix: wallet: Increment "update counter" when modifying account stuff (Luke Dashjr)
f28eb80 Bugfix: wallet: Increment "update counter" only after actually making the applicable db changes to avoid potential races (Luke Dashjr)

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