init: Use size_t consistently for cache sizes

This avoids having to rely on implicit casts when passing them to the
various functions allocating the caches.

This also ensures that if the requested amount of db_cache does not fit
in a size_t, it is clamped to the maximum value of a size_t.

Also take this opportunity to make the total amounts of cache in the
chainstate manager a size_t too.

src/test/util/setup_common.cpp

@@ -252,7 +252,7 @@ ChainTestingSetup::ChainTestingSetup(const ChainType chainType, TestOpts opts)
         LOCK(m_node.chainman->GetMutex());
         m_node.chainman->m_blockman.m_block_tree_db = std::make_unique<BlockTreeDB>(DBParams{
             .path = m_args.GetDataDirNet() / "blocks" / "index",
-            .cache_bytes = static_cast<size_t>(m_kernel_cache_sizes.block_tree_db),
+            .cache_bytes = m_kernel_cache_sizes.block_tree_db,
             .memory_only = true,
         });
     };