Eagerly flush as much data as possible from write queue

This commit implements a policy of writing as many queued buffers as the
writable socket allows, until non-writable, or the write queue is empty.
This has two objectives: 1) any allocated buffers for data copies will
be freed sooner, 2) the write output will be more responsive, fixing at
least one known delay in WiFi scanlist output in YUBOX Now.

src/AsyncTCP.h

@@ -184,6 +184,12 @@ class AsyncClient : public AsyncSocketBase
                           // If false, app owns the memory and should ensure it remains valid until acked
     } queued_writebuf;
 
+    // Internal struct used to implement sent buffer notification
+    typedef struct {
+      uint32_t length;
+      uint32_t delay;
+    } notify_writebuf;
+
     // Queue of buffers to write to socket
     SemaphoreHandle_t _write_mutex;
     std::deque<queued_writebuf> _writeQueue;
@@ -200,6 +206,8 @@ class AsyncClient : public AsyncSocketBase
     void _removeAllCallbacks(void);
     bool _flushWriteQueue(void);
     void _clearWriteQueue(void);
+    void _collectNotifyWrittenBuffers(std::deque<notify_writebuf> &, int &);
+    void _notifyWrittenBuffers(std::deque<notify_writebuf> &, int);
 
     friend void _tcpsock_dns_found(const char * name, struct ip_addr * ipaddr, void * arg);
 };