Switch from epee http server to boost::beast http server. Min boost 1.70 (#136)

There is roughly a 7.4% increase in performance in the switch to
boost::beast. Additionally, the REST endpoints `/daemon_status`,
`/get_unspent_outs`, and `/submit_raw_tx` do not block in ZMQ calls,
allowing for better response times regardless of `monerod` status.

The REST endpoints `/login and `/get_random_outs` still need updates
to prevent blocking (`/login` is conditional on DB state).

src/net/zmq_async.cpp

@@ -0,0 +1,103 @@
+// Copyright (c) 2024, The Monero Project
+// All rights reserved.
+//
+// Redistribution and use in source and binary forms, with or without modification, are
+// permitted provided that the following conditions are met:
+//
+// 1. Redistributions of source code must retain the above copyright notice, this list of
+//    conditions and the following disclaimer.
+//
+// 2. Redistributions in binary form must reproduce the above copyright notice, this list
+//    of conditions and the following disclaimer in the documentation and/or other
+//    materials provided with the distribution.
+//
+// 3. Neither the name of the copyright holder nor the names of its contributors may be
+//    used to endorse or promote products derived from this software without specific
+//    prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY
+// EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
+// MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL
+// THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
+// PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+// INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
+// STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF
+// THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+#include "zmq_async.h"
+
+#include <stdexcept>
+
+namespace net { namespace zmq
+{
+  const boost::system::error_category& boost_error_category() noexcept
+  {
+    struct category final : boost::system::error_category
+    {
+      virtual const char* name() const noexcept override final
+      {
+        return "error::error_category()";
+      }
+
+      virtual std::string message(int value) const override final
+      {
+        char const* const msg = zmq_strerror(value);
+        if (msg)
+          return msg;
+        return "zmq_strerror failure";
+      }
+
+      virtual boost::system::error_condition default_error_condition(int value) const noexcept override final
+      {
+        // maps specific errors to generic `std::errc` cases.
+        switch (value)
+        {
+        case EFSM:
+        case ETERM:
+          break;
+        default:
+          /* zmq is using cerrno errors. C++ spec indicates that `std::errc`
+            values must be identical to the cerrno value. So just map every zmq
+            specific error to the generic errc equivalent. zmq extensions must
+            be in the switch or they map to a non-existent errc enum value. */
+          return boost::system::errc::errc_t(value);
+        }
+        return boost::system::error_condition{value, *this};
+      }
+    };
+    static const category instance{};
+    return instance;
+  }
+
+  boost::system::error_code make_error_code(std::error_code code)
+  {
+    if (std::addressof(code.category()) != std::addressof(error_category()))
+      throw std::logic_error{"Expected only ZMQ errors"};
+    return boost::system::error_code{code.value(), boost_error_category()};
+  }
+
+  void free_descriptor::operator()(adescriptor* ptr) noexcept
+  {
+    if (ptr)
+    {
+      ptr->release(); // release ASIO ownership, destroys all queued handlers
+      delete ptr;
+    }
+  }
+
+  expect<async_client> async_client::make(boost::asio::io_service& io, socket zsock)
+  {
+    MONERO_PRECOND(zsock != nullptr);
+
+    int fd = 0;
+    std::size_t length = sizeof(fd);
+    if (zmq_getsockopt(zsock.get(), ZMQ_FD, &fd, &length) != 0)
+      return net::zmq::get_error_code();
+
+    async_client out{std::move(zsock), nullptr, false};
+    out.asock.reset(new adescriptor{io, fd});
+    return out;
+  }
+}} // net // zmq
+