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/rpc/fees.cpp

@@ -89,7 +89,7 @@ static RPCHelpMan estimatesmartfee()
             FeeCalculation feeCalc;
             CFeeRate feeRate{fee_estimator.estimateSmartFee(conf_target, &feeCalc, conservative)};
             if (feeRate != CFeeRate(0)) {
-                CFeeRate min_mempool_feerate{mempool.GetMinFee(gArgs.GetIntArg("-maxmempool", DEFAULT_MAX_MEMPOOL_SIZE_MB) * 1000000)};
+                CFeeRate min_mempool_feerate{mempool.GetMinFee()};
                 CFeeRate min_relay_feerate{::minRelayTxFee};
                 feeRate = std::max({feeRate, min_mempool_feerate, min_relay_feerate});
                 result.pushKV("feerate", ValueFromAmount(feeRate.GetFeePerK()));