Updates the Logger interface to allow extensibility and type erasure (#273)

Removes all the logger::on_xxx functions
Removes the Logger template parameter to async_run
Adds a logger constructor that allows passing a std::function to customize logging behavior
Adds constructors to connection and basic_connection taking a logger
Deprecates config::logger_prefix
Deprecates the async_run overload taking a logger parameter
Deprecates the basic_connection::async_run overload not taking any config object
Deprecates the basic_connection::next_layer_type typedef
Makes the default log level logger::info
Makes the logging thread-safe
Cleans up deprecated functionality from examples
Adds docs on logging
Adds an example on how to integrate spdlog into Boost.Redis logging

close #213

example/cpp20_echo_server.cpp

@@ -60,7 +60,7 @@ auto co_main(config cfg) -> asio::awaitable<void>
    auto ex = co_await asio::this_coro::executor;
    auto conn = std::make_shared<connection>(ex);
    asio::co_spawn(ex, listener(conn), asio::detached);
-   conn->async_run(cfg, {}, asio::consign(asio::detached, conn));
+   conn->async_run(cfg, asio::consign(asio::detached, conn));
 
    signal_set sig_set(ex, SIGINT, SIGTERM);
    co_await sig_set.async_wait();