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/wallet.cpp

@@ -1287,7 +1287,6 @@ void CWallet::UpdatedBlockTip()
 
 
 void CWallet::BlockUntilSyncedToCurrentChain() {
-    AssertLockNotHeld(cs_main);
     AssertLockNotHeld(cs_wallet);
 
     {