Added shared subscription support preparation code.

[redboltz]

Broker module files are separated by responsibility.
Fixed invalid memory access on endpoint_t.

mqtt_cpp/include/mqtt/broker/broker.hpp

Lines 1186 to 1205 in f659bee

	e.con.reset();
	if (e.session_expiry_interval && e.session_expiry_interval.value() != std::chrono::seconds(session_never_expire)) {
	e.tim_session_expiry = std::make_shared<as::steady_timer>(ioc_, e.session_expiry_interval.value());
	e.tim_session_expiry->async_wait(
	[this, wp = std::weak_ptr<as::steady_timer>(e.tim_session_expiry)](error_code ec) {
	if (auto sp = wp.lock()) {
	if (!ec) {
	sessions_.get<tag_tim>().erase(sp);
	}
	}
	}
	);
	}
	// TopicAlias lifetime is the same as Session lifetime
	// It is different from MQTT v5 spec but practical choice.
	// See
	// https://lists.oasis-open.org/archives/mqtt-comment/202009/msg00000.html
	e.topic_alias_recv = ep.get_topic_alias_recv_container();

[codecov]

Codecov Report

Merging #752 (1aeb108) into master (45af140) will decrease coverage by 0.07%.
The diff coverage is 88.91%.

@@            Coverage Diff             @@
##           master     #752      +/-   ##
==========================================
- Coverage   85.59%   85.52%   -0.08%     
==========================================
  Files          50       60      +10     
  Lines        8363     8557     +194     
==========================================
+ Hits         7158     7318     +160     
- Misses       1205     1239      +34     

[redboltz]

I implemented share name based shared subscriptions.

s1: $share/sn1/t1
s1: $share/sn1/t2
s2: $share/sn1/t1
s3: $share/sn1/t1
s3: $share/sn1/t2

A publisher publishes t1,t2,t1,t2.... continuously.

client	share name	topic	expected delivery order
s1	sn1	t1	1,7,9
s1	sn1	t2	4
s2	sn1	t2	2,6,10
s3	sn1	t1	3,5,11
s3	sn1	t2	8

I wrote test the step1 to 8, then it works as I expected.

I think that this PR is good start point.
After I merged the PR, I will add strategy switching mechanism.

The concept is quite simple.

class shared_target_concept {
public:
    void insert(buffer share_name, buffer topic_filter, session_state& ss);
    void erase(buffer share_name, buffer topic_filter, session_state const& ss);
    void erase(session_state const& ss);
    optional<session_state_ref> get_target(buffer const& share_name, buffer const& topic_filter);
};

The class that implements custom strategy need to implement above four member functions.
I don't decide yet how to implement the concept switcher. e.g. Template base, virtual function base, others...

In addition, I noticed that there are some protocol errors that are related to shared subscriptions.
For example, the combination of NL bit 1 and share name is protocol error.
In this PR, parsing share name is at the broker side, but perhaps it should be endpoint.hpp (process_subscribe).

NOTE: endpoint.hpp would be separated by responsibility like as broker in the PR, but it is a different topic.