Remove access to node globals from wallet-linked code

Remove last few instances of accesses to node global variables from wallet
code. Also remove accesses to node globals from code in policy/policy.cpp that
isn't actually called by wallet code, but does get linked into wallet code.

This is the last change needed to allow bitcoin-wallet tool to be linked
without depending on libbitcoin_server.a, to ensure wallet code doesn't access
node global state and avoid bugs like
https://github.com/bitcoin/bitcoin/pull/15557#discussion_r267735431

src/wallet/feebumper.cpp

@@ -174,7 +174,7 @@ Result CreateTransaction(const CWallet* wallet, const uint256& txid, const CCoin
     // This may occur if the user set TotalFee or paytxfee too low, if fallbackfee is too low, or, perhaps,
     // in a rare situation where the mempool minimum fee increased significantly since the fee estimation just a
     // moment earlier. In this case, we report an error to the user, who may use total_fee to make an adjustment.
-    CFeeRate minMempoolFeeRate = mempool.GetMinFee(gArgs.GetArg("-maxmempool", DEFAULT_MAX_MEMPOOL_SIZE) * 1000000);
+    CFeeRate minMempoolFeeRate = wallet->chain().mempoolMinFee();
     if (nNewFeeRate.GetFeePerK() < minMempoolFeeRate.GetFeePerK()) {
         errors.push_back(strprintf(
             "New fee rate (%s) is lower than the minimum fee rate (%s) to get into the mempool -- "