lnhtlc: add lock to make methods thread-safe
lnhtlc: add lock to make methods thread-safe

many methods are accessed from both the asyncio thread and the GUI thread

fixes #6373

spesmilo/electrum,pooler/electrum-ltc,spesmilo/electrum,pooler/electrum-ltc,pooler/electrum-ltc,spesmilo/electrum,wakiyamap/electrum-mona,wakiyamap/electrum-mona,spesmilo/electrum,wakiyamap/electrum-mona,wakiyamap/electrum-mona,pooler/electrum-ltc