Coin selection
--------------

### Reuse Avoidance

A new wallet flag `avoid_reuse` has been added (default off). When enabled,
a wallet will distinguish between used and unused addresses, and default to not
use the former in coin selection.

(Note: rescanning the blockchain is required, to correctly mark previously
used destinations.)

Together with "avoid partial spends" (present as of Bitcoin v0.17), this
addresses a serious privacy issue where a malicious user can track spends by
peppering a previously paid to address with near-dust outputs, which would then
be inadvertently included in future payments.

New RPCs
--------

- A new `setwalletflag` RPC sets/unsets flags for an existing wallet.


Updated RPCs
------------

Several RPCs have been updated to include an "avoid_reuse" flag, used to control
whether already used addresses should be left out or included in the operation.
These include:

- createwallet
- getbalance
- sendtoaddress

In addition, `sendtoaddress` has been changed to enable `-avoidpartialspends` when
`avoid_reuse` is enabled.

The listunspent RPC has also been updated to now include a "reused" bool, for nodes
with "avoid_reuse" enabled.