Change EventLoop::m_task_set to not use boost

EventLoop::m_task_set was unnecessary defined as
boost::optional<kj::TaskSet> because it is always initialized.

It can, however, possibly be destroyed twice: in the EventLoop::loop()
method (conditional) and in the EventLoop destructor when it goes out of
scope (unconditional). So, use std::unique_ptr to handle this.

include/mp/proxy-io.h

@@ -15,6 +15,7 @@
 #include <capnp/rpc-twoparty.h>
 
 #include <functional>
+#include <memory>
 #include <string>
 
 namespace mp {
@@ -199,7 +200,7 @@ class EventLoop
     LoggingErrorHandler m_error_handler{*this};
 
     //! Capnp list of pending promises.
-    boost::optional<kj::TaskSet> m_task_set;
+    std::unique_ptr<kj::TaskSet> m_task_set;
 
     //! List of connections.
     std::list<Connection> m_incoming_connections;

src/mp/proxy.cpp

@@ -123,13 +123,16 @@ void Connection::addAsyncCleanup(std::function<void()> fn)
 }
 
 EventLoop::EventLoop(const char* exe_name, LogFn log_fn, void* context)
-    : m_exe_name(exe_name), m_io_context(kj::setupAsyncIo()), m_log_fn(std::move(log_fn)), m_context(context)
+    : m_exe_name(exe_name),
+      m_io_context(kj::setupAsyncIo()),
+      m_log_fn(std::move(log_fn)),
+      m_context(context),
+      m_task_set(new kj::TaskSet(m_error_handler))
 {
     int fds[2];
     KJ_SYSCALL(socketpair(AF_UNIX, SOCK_STREAM, 0, fds));
     m_wait_fd = fds[0];
     m_post_fd = fds[1];
-    m_task_set.emplace(m_error_handler);
 }
 
 EventLoop::~EventLoop()