Add support for JSON batch requests

[canepat]

Specification: JSON Batch spec Comparison: Erigon RPCDaemon batch issue

Implementation hints:

• Request request_ -> std::vector<Request> requests_ in Connection
• Request& req -> std::vector<Request>& requests in RequestParser::parse
• RequestParser::consume using i-th Request as long as result is not indeterminate and begin != end (Parser::reset before each iteration)
• asio::co_spawn to launch separate coroutine stacks in Connection::do_read, one for each Request