Number of performance optimizations to subcription_map and retained_topic_map

[kleunen]

No description provided.

[redboltz]

CI error reported.

https://github.com/redboltz/mqtt_cpp/pull/691/checks?check_run_id=1269138667#step:6:32

Getting value function of MQTT_NS::optional is value() not get().

MQTT_NS::optional could be std::optional or boost::optional. Both have value() function.
boost::optional used to only have get(), but now (at least mqtt_cpp requirement version of boost), it also have value() to meet std::optional.
On the other hand std::optional doesn't have get(). So value() is good choice.

[redboltz]

I will fix it during merge process.

[redboltz]

See added commits https://github.com/redboltz/mqtt_cpp/commits/master