kernel: Move block tree db open to BlockManager constructor

Make the block db open RAII style by calling it in the BlockManager
constructor.

Before this change the block tree db was needlessly re-opened during
startup when loading a completed snapshot. Improve this by letting the
block manager open it on construction. This also simplifies the test
code a bit.

The change was initially motivated to make it easier for users of the
kernel library to instantiate a BlockManager that may be used to read
data from disk without loading the block index into a cache.

src/test/util/setup_common.cpp

@@ -257,8 +257,6 @@ ChainTestingSetup::ChainTestingSetup(const ChainType chainType, TestOpts opts)
             },
         };
         m_node.chainman = std::make_unique<ChainstateManager>(*Assert(m_node.shutdown_signal), chainman_opts, blockman_opts);
-        LOCK(m_node.chainman->GetMutex());
-        m_node.chainman->m_blockman.m_block_tree_db = m_node.chainman->m_blockman.MakeBlockTreeDb();
     };
     m_make_chainman();
 }