* core,eth: add api debug_getTrieFlushInterval
Signed-off-by: jsvisa <delweng@gmail.com>

* eth/api_debug: comment of SetTrieFlushInterval
Signed-off-by: jsvisa <delweng@gmail.com>

* Apply suggestions from code review

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Signed-off-by: jsvisa <delweng@gmail.com>
Co-authored-by: Martin Holst Swende <martin@swende.se>