Use cluster size limit for -maxmempool bound, and allow -maxmempool=0 in general

Previously we would sanity check the -maxmempool configuration based on a
multiple of the descendant size limit, but with cluster mempool the maximum
evicted size is now the cluster size limit, so use that instead.

Also allow -maxmempool=0 in general (and not just if
-limitdescendantsize/-limitclustersize is set to 0).

src/txmempool.cpp

@@ -164,9 +164,9 @@ CTxMemPool::setEntries CTxMemPool::CalculateMemPoolAncestors(const CTxMemPoolEnt
 static CTxMemPool::Options&& Flatten(CTxMemPool::Options&& opts, bilingual_str& error)
 {
     opts.check_ratio = std::clamp<int>(opts.check_ratio, 0, 1'000'000);
-    int64_t descendant_limit_bytes = opts.limits.descendant_size_vbytes * 40;
-    if (opts.max_size_bytes < 0 || opts.max_size_bytes < descendant_limit_bytes) {
-        error = strprintf(_("-maxmempool must be at least %d MB"), std::ceil(descendant_limit_bytes / 1'000'000.0));
+    int64_t cluster_limit_bytes = opts.limits.cluster_size_vbytes * 40;
+    if (opts.max_size_bytes < 0 || (opts.max_size_bytes > 0 && opts.max_size_bytes < cluster_limit_bytes)) {
+        error = strprintf(_("-maxmempool must be at least %d MB"), std::ceil(cluster_limit_bytes / 1'000'000.0));
     }
     return std::move(opts);
 }

test/functional/feature_dbcrash.py

@@ -53,7 +53,6 @@ class ChainstateWriteCrashTest(BitcoinTestFramework):
 
         # Set -maxmempool=0 to turn off mempool memory sharing with dbcache
         self.base_args = [
-            "-limitdescendantsize=0",
             "-maxmempool=0",
             "-dbbatchsize=200000",
         ]