Merge #13756: wallet: "avoid_reuse" wallet flag for improved privacy

5ebc6b0eb2 bitcoind: update -avoidpartialspends description to account for auto-enable for avoid_reuse wallets (Karl-Johan Alm)
ada258f8c8 doc: release notes for avoid_reuse (Karl-Johan Alm)
27669551da wallet: enable avoid_partial_spends by default if avoid_reuse is set (Karl-Johan Alm)
8f2e208f7c test: add test for avoidreuse feature (Karl-Johan Alm)
0bdfbd34cf wallet/rpc: add 'avoid_reuse' option to RPC commands (Karl-Johan Alm)
f904723e0d wallet/rpc: add setwalletflag RPC and MUTABLE_WALLET_FLAGS (Karl-Johan Alm)
8247a0da3a wallet: enable avoid_reuse feature (Karl-Johan Alm)
eec15662fa wallet: avoid reuse flags (Karl-Johan Alm)
58928098c2 wallet: make IsWalletFlagSet() const (Karl-Johan Alm)
129a5bafd9 wallet: rename g_known_wallet_flags constant to KNOWN_WALLET_FLAGS (Karl-Johan Alm)

Pull request description:

  Add a new wallet flag called `avoid_reuse` which, when enabled, will keep track of when a specific destination has been spent from, and will actively "blacklist" any new UTXOs which send to an already-spent-from destination.

  This improves privacy, as a payer could otherwise begin tracking a payee's wallet by regularly peppering a known UTXO with dust outputs, which would then be scooped up and used in payments by the payee, allowing the payer to map out (1) the inputs owned by the payee and (2) the destinations to which the payee is making payments.

  This replaces #10386 and together with the (now merged) #12257 it addresses #10065 in full. The concerns raised in https://github.com/bitcoin/bitcoin/pull/10386#issuecomment-302361381 are also addressed due to #12257.

  ~~Note: this builds on top of #15780.~~ (merged)

ACKs for commit 5ebc6b:
  jnewbery:
    ACK 5ebc6b0eb
  laanwj:
    Concept and code-review ACK 5ebc6b0eb2
  meshcollider:
    Code review ACK 5ebc6b0eb2
  achow101:
    ACK 5ebc6b0eb2 modulo above nits

Tree-SHA512: fdef45826af544cbbb45634ac367852cc467ec87081d86d08b53ca849e588617e9a0a255b7e7bb28692d15332de58d6c3d274ac003355220e4213d7d9070742e

src/script/ismine.h

@@ -20,7 +20,9 @@ enum isminetype
     ISMINE_NO         = 0,
     ISMINE_WATCH_ONLY = 1 << 0,
     ISMINE_SPENDABLE  = 1 << 1,
+    ISMINE_USED       = 1 << 2,
     ISMINE_ALL        = ISMINE_WATCH_ONLY | ISMINE_SPENDABLE,
+    ISMINE_ALL_USED   = ISMINE_ALL | ISMINE_USED,
     ISMINE_ENUM_ELEMENTS,
 };
 /** used for bitflags of isminetype */