[BUG] Potential leak of TSBPD thread

[maxsharabayko]

The join() operation on the TSBPD thread may be missed if CUDT::releaseSynch() finishes its execution while the receiving thread is still active and is only about to create this thread (see StartThread(m_RcvTsbPdThread, CUDT::tsbpd, ...) in CUDT::processData(..))..

It can only happen if a socket was closed right before the very first DATA packet has arrived.
The issue does not happen if the reception has already started.

This issue was reported through #1606.

The potential TSBPD thread leak existed in previous versions as well and is not something introduced in v1.4.2.
However, only C++11 threads throw an exception from the destructor if the thread is still joinable (see ~thread()).

On Windows:

• When C++11 threads are used, the std::thread's destructor throws an exception if the thread is still joinable (no one waited for it to complete).
• When pthread wrapping library is used, no exception is thrown. Hopefully, the leaking thread is then cleaned up by pthread's garbage collector.

On Linux:

• Using C++11 thread throws an exception if thread leaked (-DENABLE_STDCXX_SYNC=ON).
• Using pthreads does not throw an exception (-DENABLE_STDCXX_SYNC=OFF).

void CUDT::releaseSynch()
{
    // ...

    enterCS(m_RecvDataLock);
    if (m_RcvTsbPdThread.joinable())
    {
        m_RcvTsbPdThread.join();
    }
    leaveCS(m_RecvDataLock);
    // ...
}