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/node/interfaces.cpp

@@ -685,7 +685,7 @@ public:
     CFeeRate mempoolMinFee() override
     {
         if (!m_node.mempool) return {};
-        return m_node.mempool->GetMinFee(gArgs.GetIntArg("-maxmempool", DEFAULT_MAX_MEMPOOL_SIZE_MB) * 1000000);
+        return m_node.mempool->GetMinFee();
     }
     CFeeRate relayMinFee() override { return ::minRelayTxFee; }
     CFeeRate relayIncrementalFee() override { return ::incrementalRelayFee; }