Make a writem API with just iovecs [5/n]

Summary:
**Background for the diff stack**: We'd like to excise the usage of folly from a library that we maintain (quic), which is why we'd like to have a version of `writemGSO` that takes in iovecs instead of IOBufs.

**What this diff is doing**: We're creating a new `writemv` function that does basically the same thing that `writem` does, except it uses `iovec`s instead of `IOBuf`s. This is pretty similar to `writem`.

Reviewed By: mjoras

Differential Revision: D68736205

fbshipit-source-id: b6dc9b1740d31bff97ec9e0b49923717f0b819b5

folly/io/async/AsyncUDPSocket.cpp

@@ -865,6 +865,14 @@ int AsyncUDPSocket::writemGSO(
   return ret;
 }
 
+int AsyncUDPSocket::writemv(
+    Range<SocketAddress const*> addrs,
+    iovec* iov,
+    size_t* numIovecsInBuffer,
+    size_t count) {
+  return writemGSOv(addrs, iov, numIovecsInBuffer, count, nullptr);
+}
+
 int AsyncUDPSocket::writemGSOv(
     Range<SocketAddress const*> addrs,
     iovec* iov,

folly/io/async/AsyncUDPSocket.h

@@ -270,6 +270,19 @@ class AsyncUDPSocket : public EventHandler {
       const std::unique_ptr<folly::IOBuf>* bufs,
       size_t count);
 
+  /**
+   * Send the data in buffers to destination. Returns the return code from
+   * ::sendmmsg.
+   * iov is an array of iovecs, which is composed of "count" messages that
+   * need to be sent. Each message can have multiple iovecs. The number of
+   * iovecs per message is specified in numIovecsInBuffer.
+   */
+  virtual int writemv(
+      Range<SocketAddress const*> addrs,
+      iovec* iov,
+      size_t* numIovecsInBuffer,
+      size_t count);
+
   /**
    * Send the data in buffer to destination. Returns the return code from
    * ::sendmsg.