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_json.cpp

@@ -58,7 +58,7 @@ auto co_main(config cfg) -> asio::awaitable<void>
 {
    auto ex = co_await asio::this_coro::executor;
    auto conn = std::make_shared<connection>(ex);
-   conn->async_run(cfg, {}, asio::consign(asio::detached, conn));
+   conn->async_run(cfg, asio::consign(asio::detached, conn));
 
    // user object that will be stored in Redis in json format.
    user const u{"Joao", "58", "Brazil"};