Simplify semantics of ChainStateFlushed callback

Summary:
Previously, ChainStateFlushed would fire either if a full flush
completed (which can happen due to memory limits, forced flush, or
on its own DATABASE_WRITE_INTERVAL timer) *or* on a
ChainStateFlushed-specific DATABASE_WRITE_INTERVAL timer. This is
both less clear for clients (as there are no guarantees about a
flush having actually happened prior to the call), and reults in
extra flushes not clearly intended by the code. We drop the second
case, providing a strong guarantee without removing the periodit
timer-based flushing.

backport of second commit (9cb6cdc) from PR13106
Depends on D2927

Test Plan: make check and functional tests

Reviewers: #bitcoin_abc, deadalnix, Fabien

Reviewed By: #bitcoin_abc, Fabien

Maniphest Tasks: T632

Differential Revision: https://reviews.bitcoinabc.org/D2928