rpc: Push down safe mode checks

Summary:
This contains most of the changes of 10563 "remove safe mode", but doesn't
remove the safe mode yet, but put an ObserveSafeMode() check in
individual calls with okSafeMode=false.

This cleans up the ugly "okSafeMode" flag from the dispatch tables,
which is not a concern for the RPC server.

Extra-author: Wladimir J. van der Laan <laanwj@gmail.com>

This is a backport of Core PR11179

Test Plan:

make check
./test/functional/test_runner.py

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1917