[rpc] Undeprecate estimatefee

Summary:
Revert "[rpc] Deprecate estimatefee RPC" (D1920)

This reverts PARTIALLY reverts commit 49ff27fedd365dcf5f7639fd2bda5dfc3cc58f51

The instrumentation for enabling deprecated RPCs is left in place for future use.
However, estimatefee should not be deprecated in a point release. It's also possible
that it should not be deprecated at all since we removed estimatesmartefee which
core deprecated this in favor of.

Depends on D2013

Test Plan:

make check && ./test/functional/test_runner.py

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, deadalnix, jasonbcox

Subscribers: jasonbcox, deadalnix, teamcity

Differential Revision: https://reviews.bitcoinabc.org/D2012