fix dtls multi-thread crash issue

[yuanchao0310]

Description

fix dtls multi-thread crash issue:
refer to:
https://linux.die.net/man/3/crypto_set_locking_callback
http://curl.haxx.se/libcurl/c/opensslthreadlock.html
http://curl.haxx.se/libcurl/c/threaded-ssl.html

[yuanchao0310]

Sample code to reproduce the crash bug, it similar with the constructor of DtlsTransport.cpp (erizo/src/erizo/DtlsTransport.cpp)

#include "dtls/DtlsSocket.h"
#include <thread>

const int thread_num = 100;

void func() {
  boost::scoped_ptr<dtls::DtlsSocketContext> dtlsRtp;
  dtlsRtp.reset(new dtls::DtlsSocketContext());
  dtlsRtp->createClient();
}

int main()
{
  dtls::DtlsSocketContext::Init();
  std::thread th[thread_num];
  for (int i = 0; i < 100; i++ ) {
    for (int j = 0; j < thread_num; j++) {
      th[j] = std::thread(func);
    }
    for (int j = 0; j < thread_num; j++) {
      th[j].join();
    }
  }

  return 0;
}

the coredump backtrace looks like:
#0 0x0000000001af7c68 in rsa_pub_cmp ()
#1 0x0000000001a77d81 in X509_check_private_key ()
#2 0x00000000019fb793 in SSL_CTX_use_PrivateKey ()
#3 0x0000000000c0a06a in dtls::DtlsSocketContext::DtlsSocketContext (this=0x7fccccffe000) at /opt/download/licode/erizo/src/erizo/dtls/DtlsClient.cpp

[yuanchao0310]

Another example code from ffmpeg:
https://github.com/FFmpeg/FFmpeg/blob/master/libavformat/tls_openssl.c

[jcague]

@yuanchao0310 I submitted a proposal with changes for this PR, including a unit test: yuanchao0310#1

[yuanchao0310]

@yuanchao0310 I submitted a proposal with changes for this PR, including a unit test: yuanchao0310#1

Merged. Please review again.

[jcague]

LGTM, thanks a lot for this contribution @yuanchao0310 !