mempool: Pass in -maxmempool instead of referencing gArgs

- Store the mempool size limit (-maxmempool) in CTxMemPool as a member.

- Remove the requirement to explicitly specify a mempool size limit for
  CTxMemPool::GetMinFee(...) and LimitMempoolSize(...), just use the
  stored mempool size limit where possible.

- Remove all now-unnecessary instances of:
    gArgs.GetIntArg("-maxmempool", DEFAULT_MAX_MEMPOOL_SIZE_MB) * 1000000

The code change in CChainState::GetCoinsCacheSizeState() is correct
since the coinscache should not repurpose "extra" mempool memory
headroom for itself if the mempool doesn't even exist.

src/kernel/mempool_options.h

@@ -4,8 +4,13 @@
 #ifndef BITCOIN_KERNEL_MEMPOOL_OPTIONS_H
 #define BITCOIN_KERNEL_MEMPOOL_OPTIONS_H
 
+#include <cstdint>
+
 class CBlockPolicyEstimator;
 
+/** Default for -maxmempool, maximum megabytes of mempool memory usage */
+static constexpr unsigned int DEFAULT_MAX_MEMPOOL_SIZE_MB{300};
+
 namespace kernel {
 /**
  * Options struct containing options for constructing a CTxMemPool. Default
@@ -19,6 +24,7 @@ struct MemPoolOptions {
     CBlockPolicyEstimator* estimator{nullptr};
     /* The ratio used to determine how often sanity checks will run.  */
     int check_ratio{0};
+    int64_t max_size_bytes{DEFAULT_MAX_MEMPOOL_SIZE_MB * 1'000'000};
 };
 } // namespace kernel