[core] Fix TSBPD thread create/join protection.

[maxsharabayko]

Extracted from PR #2893.

WARNING: ThreadSanitizer: data race (pid=5444)
  Write of size 8 at 0x7ba80005f118 by thread T1 (mutexes: write M226, write M223, write M898885562122236320, write M898604087145525592):
    #0 srt::sync::CThread::join() srt/srtcore/sync_posix.cpp:442 (srt-xtransmit+0x464f2a)
    #1 srt::CUDT::releaseSynch() srt/srtcore/core.cpp:7839 (srt-xtransmit+0x34d600)
    #2 srt::CUDT::closeInternal() srt/srtcore/core.cpp:6332 (srt-xtransmit+0x34c769)
    #3 srt::CUDTSocket::breakSocket_LOCKED() srt/srtcore/api.cpp:127 (srt-xtransmit+0x28d9e1)
    #4 srt::CUDTUnited::garbageCollect(void*) srt/srtcore/api.cpp:3328 (srt-xtransmit+0x28f180)

  Previous read of size 8 at 0x7ba80005f118 by thread T4:
    #0 srt::sync::CThread::joinable() const srt/srtcore/sync_posix.cpp:424 (srt-xtransmit+0x464ce9)
    #1 srt::CUDT::checkLazySpawnTsbPdThread() srt/srtcore/core.cpp:9997 (srt-xtransmit+0x371cd6)
    #2 srt::CUDT::processData(srt::CUnit*) srt/srtcore/core.cpp:10293 (srt-xtransmit+0x376f6b)
    #3 srt::CRcvQueue::worker_ProcessAddressedPacket(int, srt::CUnit*, srt::sockaddr_any const&) srt/srtcore/queue.cpp:1475 (srt-xtransmit+0x3f01f6)
    #4 srt::CRcvQueue::worker(void*) srt/srtcore/queue.cpp:1254 (srt-xtransmit+0x3ecfb7)