When the mempool fills the verbose `getrawmempool` RPC call gets really slow.  It can take minutes for >100k transactions in the mempool.

The culprit is the univalue library.  Objects are implemented as a big list, but the library checks for duplicate keys.  So inserting n key-value pairs into an object takes O(n^2).

Possible fixes:

- Improve univalue library by using linked hash maps.
- change the RPC output format of getrawmempool to use a list of objects, instead of an object of objects.  But this breaks existing code.
- hack the library to be able to skip the duplicate check.

RPCHelpMan{"preciousblock",
                "\nTreats a block as if it were received before others with the same work.\n"
                "\nA later preciousblock call can override the effect of an earlier one.\n"
                "\nThe effects of preciousblock are not retained across restarts.\n",
                {
                    {"blockhash", RPCArg::Type::STR_HEX, RPCArg::Optional::NO, "the hash of the block to mark as precious"},
                },
                RPCResults{},
                RPCExamples{
                    HelpExampleCli("preciousblock", "\"blockhash\"")
            + HelpExampleRpc("preciousblock", "\"blockhash\"")
                },
            }.ToString());
    uint256 hash(ParseHashV(request.params[0], "blockhash"));
    CBlockIndex* pblockindex;
    {
        LOCK(cs_main);
        pblockindex = LookupBlockIndex(hash);
        if (!pblockindex) {
            throw JSONRPCError(RPC_INVALID_ADDRESS_OR_KEY, "Block not found");
        }
    }
    CValidationState state;
    PreciousBlock(state, Params(), pblockindex);
    if (!state.IsValid()) {
        throw JSONRPCError(RPC_DATABASE_ERROR, FormatStateMessage(state));
    }
    return NullUniValue;
}
static UniValue invalidateblock(const JSONRPCRequest& request)