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>

test/functional/feature_init.py

@@ -55,7 +55,6 @@ class InitStressTest(BitcoinTestFramework):
             b'Loading P2P addresses',
             b'Loading banlist',
             b'Loading block index',
-            b'Switching active chainstate',
             b'Checking all blk files are present',
             b'Loaded best chain:',
             b'init message: Verifying blocks',