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).

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",
         ]