validation: make IsInitialBlockDownload() lock-free

`ChainstateManager::IsInitialBlockDownload()` is queried on hot paths and previously acquired `cs_main` internally, contributing to lock contention.

Cache the IBD status in `m_cached_is_ibd`, and introduce `ChainstateManager::UpdateIBDStatus()` to latch it once block loading has finished and the current chain tip has enough work and is recent.
Call the updater after tip updates and after `ImportBlocks()` completes.

Since `IsInitialBlockDownload()` no longer updates the cache, drop `mutable` from `m_cached_is_ibd` and only update it from `UpdateIBDStatus()` under `cs_main`.

Update the new unit test to showcase the new `UpdateIBDStatus()`.

Co-authored-by: Patrick Strateman <patrick.strateman@gmail.com>
Co-authored-by: Martin Zumsande <mzumsande@gmail.com>

src/test/validation_chainstatemanager_tests.cpp

@@ -183,6 +183,7 @@ BOOST_FIXTURE_TEST_CASE(chainstatemanager_ibd_exit_after_loading_blocks, ChainTe
         } else {
             assert(!chainman.ActiveChain().Tip());
         }
+        chainman.UpdateIBDStatus();
     }};
 
     for (const bool cached_is_ibd : {false, true}) {