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
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Depends on D6233
Partial backport of Core PR15639