Make etl::Publisher stoppable

[kuznetsss]

Issue Description

etl::LedgerPublisher has this loop:

clio/src/etl/impl/LedgerPublisher.hpp

Line 128 in 88881e9

while (not state_.get().isStopping) {

But the flag isStopping is not set anywhere so it is always false.
Also the method publish() is called with maxAttempts = std::nullopt:

clio/src/etl/ETLService.cpp

Line 368 in 88881e9

publisher_->publish(seq, {});

So if LedgerPublisher couldn't fetch range or range is older it will stuck in an infinite loop.

Probably we should add method stop() to LedgerPublisher and call it in ETLService::stop().