Avoid asking for user exist with a room

[NelsonVides]

Similarly to what happened to inbox in 61d90d0. This time I just extract common code and reuse it in MAM.

[codecov]

Codecov Report

Merging #3470 (3374484) into master (587b279) will decrease coverage by 0.05%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #3470      +/-   ##
==========================================
- Coverage   80.91%   80.86%   -0.06%     
==========================================
  Files         415      415              
  Lines       32272    32271       -1     
==========================================
- Hits        26114    26097      -17     
- Misses       6158     6174      +16     

Impacted Files	Coverage Δ
src/inbox/mod_inbox.erl	87.95% <100.00%> (-0.49%)	⬇️
src/mam/mod_mam.erl	88.57% <100.00%> (+0.04%)	⬆️
src/mongoose_lib.erl	84.31% <100.00%> (+2.09%)	⬆️
src/mod_roster_riak.erl	81.53% <0.00%> (-15.39%)	⬇️
src/elasticsearch/mongoose_elasticsearch.erl	76.92% <0.00%> (-7.70%)	⬇️
...bal_distrib/mod_global_distrib_hosts_refresher.erl	75.47% <0.00%> (-1.89%)	⬇️
src/mam/mod_mam_elasticsearch_arch.erl	85.08% <0.00%> (-1.76%)	⬇️
src/logger/mongoose_log_filter.erl	78.08% <0.00%> (-1.37%)	⬇️
src/ejabberd_c2s.erl	88.97% <0.00%> (-0.30%)	⬇️
src/mod_muc_room.erl	76.97% <0.00%> (-0.29%)	⬇️
... and 7 more

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[mongoose-im]

small_tests_24 / small_tests / 3374484
Reports root / small

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small_tests_23 / small_tests / 3374484
Reports root / small

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dynamic_domains_pgsql_mnesia_23 / pgsql_mnesia / 3374484
Reports root/ big
OK: 2725 / Failed: 0 / User-skipped: 186 / Auto-skipped: 0

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dynamic_domains_pgsql_mnesia_24 / pgsql_mnesia / 3374484
Reports root/ big
OK: 2725 / Failed: 0 / User-skipped: 186 / Auto-skipped: 0

---

dynamic_domains_mysql_redis_24 / mysql_redis / 3374484
Reports root/ big
OK: 2708 / Failed: 0 / User-skipped: 203 / Auto-skipped: 0

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ldap_mnesia_23 / ldap_mnesia / 3374484
Reports root/ big
OK: 1500 / Failed: 0 / User-skipped: 386 / Auto-skipped: 0

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ldap_mnesia_24 / ldap_mnesia / 3374484
Reports root/ big
OK: 1500 / Failed: 0 / User-skipped: 386 / Auto-skipped: 0

---

dynamic_domains_mssql_mnesia_24 / odbc_mssql_mnesia / 3374484
Reports root/ big
OK: 2725 / Failed: 0 / User-skipped: 186 / Auto-skipped: 0

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internal_mnesia_24 / internal_mnesia / 3374484
Reports root/ big
OK: 1585 / Failed: 0 / User-skipped: 301 / Auto-skipped: 0

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pgsql_mnesia_24 / pgsql_mnesia / 3374484
Reports root/ big
OK: 3112 / Failed: 0 / User-skipped: 195 / Auto-skipped: 0

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mysql_redis_24 / mysql_redis / 3374484
Reports root/ big
OK: 3107 / Failed: 0 / User-skipped: 200 / Auto-skipped: 0

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elasticsearch_and_cassandra_24 / elasticsearch_and_cassandra_mnesia / 3374484
Reports root/ big
OK: 1878 / Failed: 0 / User-skipped: 313 / Auto-skipped: 0

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pgsql_mnesia_23 / pgsql_mnesia / 3374484
Reports root/ big
OK: 3112 / Failed: 0 / User-skipped: 195 / Auto-skipped: 0

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mssql_mnesia_24 / odbc_mssql_mnesia / 3374484
Reports root/ big
OK: 3112 / Failed: 0 / User-skipped: 195 / Auto-skipped: 0

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riak_mnesia_24 / riak_mnesia / 3374484
Reports root/ big
OK: 1724 / Failed: 0 / User-skipped: 314 / Auto-skipped: 0