init: add utxo snapshot detection

Add functionality for activating a snapshot-based chainstate if one is
detected on-disk.

Also cautiously initialize chainstate cache usages so that we don't
somehow blow past our cache allowances during initialization, then
rebalance at the end of init.

Co-authored-by: Russell Yanofsky <russ@yanofsky.org>

src/node/utxo_snapshot.cpp

@@ -8,6 +8,7 @@
 #include <logging.h>
 #include <streams.h>
 #include <uint256.h>
+#include <util/system.h>
 #include <validation.h>
 
 #include <cstdio>
@@ -76,4 +77,15 @@ std::optional<uint256> ReadSnapshotBaseBlockhash(fs::path chaindir)
     return base_blockhash;
 }
 
+std::optional<fs::path> FindSnapshotChainstateDir()
+{
+    fs::path possible_dir =
+        gArgs.GetDataDirNet() / fs::u8path(strprintf("chainstate%s", SNAPSHOT_CHAINSTATE_SUFFIX));
+
+    if (fs::exists(possible_dir)) {
+        return possible_dir;
+    }
+    return std::nullopt;
+}
+
 } // namespace node