Some wallet calls (like getbalances) need to lock the chainstate to make sure they are caught up on the latest block (which may have been reported to the users in getblockchaininfo or similar).

Those calls could benefit if the block hash they were valid on was included in the response. For example, immature balance can only change when the chainstate changes. (Assuming no keys are imported)

It seems those calls can include the block hash they are caught up with for free in the response. They could simply include `m_last_block_processed` in the response.

I haven't checked back with other developers to see if they like this approach, but to start simple only a single call could be updated for now. E.g. `getbalances`.

#### Useful skills:

Basic understanding of threads, the Bitcoin Core wallet and the Bitcoin Core RPC interface.

#### Want to work on this issue?

The purpose of the `good first issue` label is to highlight which issues are suitable for a new contributor without a deep understanding of the codebase.

You do not need to request permission to start working on this. You are encouraged to comment on the issue if you are planning to work on it. This will help other contributors monitor which issues are actively being addressed and is also an effective way to request assistance if and when you need it.

For guidance on contributing, please read [CONTRIBUTING.md](https://github.com/bitcoin/bitcoin/blob/master/CONTRIBUTING.md) before opening your pull request.

filter_coins.min_amount = 0;
    if (!request.params[4].isNull()) {
        const UniValue& options = request.params[4].get_obj();
        RPCTypeCheckObj(options,
            {
                {"minimumAmount", UniValueType()},
                {"maximumAmount", UniValueType()},
                {"minimumSumAmount", UniValueType()},
                {"maximumCount", UniValueType(UniValue::VNUM)},
                {"include_immature_coinbase", UniValueType(UniValue::VBOOL)}
            },
            true, true);
        if (options.exists("minimumAmount"))
            filter_coins.min_amount = AmountFromValue(options["minimumAmount"]);
        if (options.exists("maximumAmount"))
            filter_coins.max_amount = AmountFromValue(options["maximumAmount"]);
        if (options.exists("minimumSumAmount"))
            filter_coins.min_sum_amount = AmountFromValue(options["minimumSumAmount"]);
        if (options.exists("maximumCount"))
            filter_coins.max_count = options["maximumCount"].getInt<int64_t>();
        if (options.exists("include_immature_coinbase")) {
            filter_coins.include_immature_coinbase = options["include_immature_coinbase"].get_bool();
        }
    }
    
    
    pwallet->BlockUntilSyncedToCurrentChain();
    UniValue results(UniValue::VARR);
    std::vector<COutput> vecOutputs;
    {