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/mempool_args.cpp

@@ -13,4 +13,6 @@ using kernel::MemPoolOptions;
 void ApplyArgsManOptions(const ArgsManager& argsman, MemPoolOptions& mempool_opts)
 {
     mempool_opts.check_ratio = argsman.GetIntArg("-checkmempool", mempool_opts.check_ratio);
+
+    if (auto mb = argsman.GetIntArg("-maxmempool")) mempool_opts.max_size_bytes = *mb * 1'000'000;
 }