Deprecate estimatefee in v0.16, for final removal in v0.17.
This commit introduces a phased removal of RPC methods. RPC method is
disabled by default in version x, but can be enabled by using the
`-deprecatedrpc=<method>` argument. RPC method is removed entirely in
version (x+1).
Use POSIX rename atomicity at the `bitcoind` side to create a working
cookie atomically:
- Write `.cookie.tmp`, close file
- Rename `.cookie.tmp` to `.cookie`
This avoids clients reading invalid/partial cookies as in #11129.
Raise RPC_WALLET_NOT_SPECIFIED instead of RPC_METHOD_NOT_FOUND when a required
wallet filename was not specified in an RPC call.
Also raise more specific RPC_WALLET_NOT_FOUND error instead of
RPC_INVALID_PARAMETER in case an invalid wallet was specified, for consistency.
RPC_INVALID_REQUEST and RPC_METHOD_NOT_FOUND are mapped internally to
HTTP error codes and should not be used for application-layer errors.
This commit adds commenting around those definitions to warn not to use
them for application errors.
