Create LcmLogSink, a VectorLogSink analog implemented as an lcm interface

[Brian-Acosta]

Usage example:

auto lcm_log_sink = LcmLogSink()
auto publishes_to_log = LcmPublisherSystem<lcmt_my_message>::Make(&lcm_log_sink, ...);

// build the diagram and set up the simulation

simulator.AdvanceTo(end_time);

lcm_log_sink.WriteLog("lcmlog-00");

This is similar to Drake's DrakeLcmLog, but avoids writing to disk for every Publish event, instead deferring until the user calls WriteLog

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This change is 

[yangwill]

Reviewed 2 of 3 files at r1, all commit messages.
Reviewable status: 2 of 3 files reviewed, 2 unresolved discussions (waiting on @Brian-Acosta)

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lcm/lcm_log_sink.cc line 88 at r1 (raw file):

  unsigned long timestamp;
  if (!overwrite_publish_time_with_system_clock_) {
    timestamp = second_to_timestamp(time_sec.value_or(0.0));

is there a better way to deal with the optional?

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lcm/lcm_log_sink.cc line 91 at r1 (raw file):

  } else {
    timestamp = std::chrono::steady_clock::now().time_since_epoch() /
        std::chrono::microseconds(1);

it feels like there should be a command that's more direct than dividing by microseconds

[yangwill]

Changes look good, just want to confirm that the implementation of the timestamps is as intended

Reviewable status: 2 of 3 files reviewed, 2 unresolved discussions (waiting on @Brian-Acosta)