Avoid leaking uv_async_t in Dispatcher::Deactivate()

On disconnect, Dispatcher::Deactivate() calls uv_close() to close its
active uv_async_t instance and nulls out the corresponding field,
but never frees the underlying memory.

To reproduce, run the following with node-rdkafka and librdkafka
compiled with ASAN:
```js
const { KafkaConsumer } = require('../');

const consumer = new KafkaConsumer({
  'group.id': 'kafka',
  'metadata.broker.list': 'localhost:9092',
}, {});

consumer.connect({ timeout: 2000 }, function (err) {
  if (err) {
    console.error('Error connecting to Kafka:', err);
    return;
  }

  consumer.disconnect();
})
```

This should report:
```
Direct leak of 128 byte(s) in 1 object(s) allocated from:
    #0 0x7f049c3aa647 in operator new(unsigned long) ../../../../src/libsanitizer/asan/asan_new_delete.cpp:99
    Blizzard#1 0x7f0471d872a1 in NodeKafka::Callbacks::Dispatcher::Activate() ../src/callbacks.cc:69
    Blizzard#2 0x7f0471e75c06 in NodeKafka::Workers::KafkaConsumerConnect::HandleOKCallback() ../src/workers.cc:579
    Blizzard#3 0x7f0471dd679c in Nan::AsyncWorker::WorkComplete() ../node_modules/nan/nan.h:2008
    Blizzard#4 0x7f0471dd679c in Nan::AsyncExecuteComplete(uv_work_s*) ../node_modules/nan/nan.h:2365
    Blizzard#5 0x7f0471dd679c in Nan::AsyncExecuteComplete(uv_work_s*, int) ../node_modules/nan/nan.h:2369
    Blizzard#6 0x18bb75c in uv__work_done ../deps/uv/src/threadpool.c:329
    Blizzard#7 0x18bf0b2 in uv__async_io ../deps/uv/src/unix/async.c:176
    Blizzard#8 0x18d3b2a in uv__io_poll ../deps/uv/src/unix/linux.c:1485
    Blizzard#9 0x18bfdd6 in uv_run ../deps/uv/src/unix/core.c:447
    Blizzard#10 0xbc9be5 in node::SpinEventLoopInternal(node::Environment*) (/usr/bin/node+0xbc9be5)
    Blizzard#11 0xd1d920 in node::NodeMainInstance::Run(node::ExitCode*, node::Environment*) [clone .part.0] (/usr/bin/node+0xd1d920)
    Blizzard#12 0xd1e38c in node::NodeMainInstance::Run() (/usr/bin/node+0xd1e38c)
    Blizzard#13 0xc710be in node::Start(int, char**) (/usr/bin/node+0xc710be)
    Blizzard#14 0x7f049bdf0d09 in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x23d09)
```

The [libuv documentation](https://docs.libuv.org/en/v1.x/handle.html#c.uv_close)
says it's only safe to free the underlying memory in a close callback
passed to uv_close(), or after such a callback has returned. So, use a
unique_ptr with a custom deleter to accomplish this.

Author: mszabo-wikia

src/callbacks.cc

@@ -48,8 +48,7 @@ v8::Local<v8::Array> TopicPartitionListToV8Array(
   return tp_array;
 }
 
-Dispatcher::Dispatcher() {
-  async = NULL;
+Dispatcher::Dispatcher(): async(nullptr, async_deleter) {
   uv_mutex_init(&async_lock);
 }
 
@@ -66,19 +65,17 @@ Dispatcher::~Dispatcher() {
 // Only run this if we aren't already listening
 void Dispatcher::Activate() {
   if (!async) {
-    async = new uv_async_t;
-    uv_async_init(uv_default_loop(), async, AsyncMessage_);
+    async = std::unique_ptr<uv_async_t, decltype(async_deleter)*>(
+      new uv_async_t(), async_deleter);
+    uv_async_init(uv_default_loop(), async.get(), AsyncMessage_);
 
     async->data = this;
   }
 }
 
 // Should be able to run this regardless of whether it is active or not
 void Dispatcher::Deactivate() {
-  if (async) {
-    uv_close(reinterpret_cast<uv_handle_t*>(async), NULL);
-    async = NULL;
-  }
+  async.reset();
 }
 
 bool Dispatcher::HasCallbacks() {
@@ -87,7 +84,7 @@ bool Dispatcher::HasCallbacks() {
 
 void Dispatcher::Execute() {
   if (async) {
-    uv_async_send(async);
+    uv_async_send(async.get());
   }
 }
 
@@ -119,6 +116,17 @@ void Dispatcher::RemoveCallback(const v8::Local<v8::Function> &cb) {
   }
 }
 
+// Custom deleter for uv_async_t smart pointers
+void Dispatcher::async_deleter(uv_async_t* async) {
+  uv_close(
+    reinterpret_cast<uv_handle_t*>(async),
+    // Release memory after uv_close() has finished.
+    [](uv_handle_t* handle) {
+      delete reinterpret_cast<uv_async_t*>(handle);
+    }
+  );
+}
+
 event_t::event_t(const RdKafka::Event &event) {
   message = "";
   fac = "";

src/callbacks.h

@@ -12,6 +12,7 @@
 #include <uv.h>
 #include <nan.h>
 
+#include <memory>
 #include <vector>
 #include <deque>
 
@@ -49,7 +50,9 @@ class Dispatcher {
      dispatcher->Flush();
   }
 
-  uv_async_t *async;
+  static inline void async_deleter(uv_async_t* async);
+
+  std::unique_ptr<uv_async_t, decltype(async_deleter)*> async;
 };
 
 struct event_t {