Mam inbox unification

[NelsonVides]

If inbox is nothing more than a low-cost summary of mam, we can expect inbox to store pretty much what is stored in mam, and nothing else.

This is currently not true, as inbox is not taking into account no-store hints for example, and it is storing body-less messages, like for example it would store carbon copies.

Additionally, make both take the <store> hint as enforcing storage.

[codecov]

Codecov Report

Merging #3165 (d99449c) into master (45c3308) will decrease coverage by 0.04%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #3165      +/-   ##
==========================================
- Coverage   80.27%   80.23%   -0.05%     
==========================================
  Files         396      396              
  Lines       32260    32251       -9     
==========================================
- Hits        25896    25875      -21     
- Misses       6364     6376      +12     

Impacted Files	Coverage Δ
src/inbox/mod_inbox.erl	90.15% <100.00%> (-0.05%)	⬇️
src/mam/mod_mam_utils.erl	89.96% <100.00%> (+0.06%)	⬆️
src/mod_roster_riak.erl	81.53% <0.00%> (-15.39%)	⬇️
src/ejabberd.erl	45.00% <0.00%> (-10.00%)	⬇️
src/mongoose_tcp_listener.erl	76.59% <0.00%> (-4.26%)	⬇️
src/mod_bosh.erl	91.89% <0.00%> (-2.03%)	⬇️
src/inbox/mod_inbox_entries.erl	96.72% <0.00%> (-1.64%)	⬇️
src/domain/service_domain_db.erl	78.12% <0.00%> (-1.57%)	⬇️
src/global_distrib/mod_global_distrib_utils.erl	64.48% <0.00%> (-0.94%)	⬇️
src/pubsub/mod_pubsub_db_mnesia.erl	92.40% <0.00%> (-0.43%)	⬇️
... and 5 more

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[chrzaszcz]

Looks good in general, I have two minor remarks.

[chrzaszcz]

Nice, it's good to see the things unified.