To reduce the performance penalty of commit 779ca42a
Ian has recommended to release the socket_mutex during the call of select.
See https://github.com/eclipse/paho.mqtt.c/issues/385#issuecomment-372299490

As a preparation for this, the locking of the socket_mutex is moved inside
the Socket_getReadySocket() function.
Signed-off-by: Juergen Kosel <juergen.kosel@softing.com>

* develop:
  Remove reference to internal Heap functions for now
  Some cleanup of testing code
  Add call to AppVeyor env setup
  Do we need spaces?
  Try again...
  Change vcvars syntax
  Check visual studio env var
  Correct vcvarsall location
  Add Visual Studio 2013 build as well as 2015
  Reinstate .travis.yml
  Update cd command
  Use HTTPS git checkout instead of SSH
  Another Python3 try
  Update Python3 version
  Try AppVeyor build with local broker
  修改了restoreCommand时：（1）topics指针未申请内存的问题；（2）unsub误写为sub的问题。
  fix issue that MQTTClient_connect hangs on network outage, for windows

# Conflicts:
#	src/Socket.c
#	test/test6.c
Signed-off-by: Juergen Kosel <juergen.kosel@softing.com>

fixd: when restore command, "topics" no malloc.

* '373' of https://github.com/eclipse/paho.mqtt.c.git:
  More cleanup when a socket is closed #373

# Conflicts:
#	src/Socket.c
Signed-off-by: Juergen Kosel <juergen.kosel@softing.com>

* '373' of https://github.com/eclipse/paho.mqtt.c.git:
  Don't try ack writes for sockets with pending output, missing connect failure call #373

* test4issue373:
  Extend test_issue373 to test also with QoS 1 and 2
  Add helper script for test_issue373 to cyclically disconnect the VM running the broker
  Add log output for failed message queueing in test due to disconnect
  Do not reduce the tcp send buffer size for production code

# Conflicts:
#	test/test_issue373.c
Signed-off-by: Juergen Kosel <juergen.kosel@softing.com>

Signed-off-by: Juergen Kosel <juergen.kosel@softing.com>

Signed-off-by: Juergen Kosel <juergen.kosel@softing.com>

Signed-off-by: Juergen Kosel <juergen.kosel@softing.com>

Signed-off-by: Juergen Kosel <juergen.kosel@softing.com>

The functions implemented in Heap.c are not usable for Windows builds.
Therefore use only for none MSVC builds.
Signed-off-by: Juergen Kosel <juergen.kosel@softing.com>

* '373' of https://github.com/eclipse/paho.mqtt.c.git:
  Add latest restart test changes
  Add test changes

It turned out in https://github.com/eclipse/paho.mqtt.c/issues/385 that
Socket_getReadySocket() could remove a list element, which is currently
in use by a function called by Socket_noPendingWirte().
And this leads to memory corruption or segmentation fault.
To fix this, this call of Socket_PendingWrites() in MQTTAsync.c needs to be protected by mutex_socket.
Signed-off-by: Juergen Kosel <juergen.kosel@softing.com>

fix issue that MQTTClient_connect hangs on network outage, for windows

Signed-off-by: Juergen Kosel <juergen.kosel@softing.com>

Setting pointers to NULL after free never hurt, but may prevent
double free and other problems.
Signed-off-by: Juergen Kosel <juergen.kosel@softing.com>

Signed-off-by: Juergen Kosel <juergen.kosel@softing.com>

The function myfree() is used as a replacement for free().
Therefore it must not cause any failure, if someone calls myfree(NULL).
Signed-off-by: Juergen Kosel <juergen.kosel@softing.com>

Signed-off-by: Juergen Kosel <juergen.kosel@softing.com>