[backport#15639] Remove access to node globals from wallet-linked code

Summary:
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

https://github.com/bitcoin/bitcoin/pull/15639/commits/b874747b51882a613895a100c4210c7f1dddde30

Depends on D6233

Partial backport of Core PR15639

Test Plan:

ninja check-all

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D6255