Make enforcing simple queries configurable

[NelsonVides]

This might be useful when we just don't want to offer clients the possibility of making very expensive queries. A conversation with over 10k messages is not uncommon, and when there are many of them and that many are fetched, it can put the DB at DDoS risk.
So if the operator desires so, all counting operations can be removed to begin with.

[codecov]

Codecov Report

Merging #3733 (28cb7a1) into master (79c8a87) will increase coverage by 0.36%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #3733      +/-   ##
==========================================
+ Coverage   81.95%   82.31%   +0.36%     
==========================================
  Files         526      526              
  Lines       33839    33844       +5     
==========================================
+ Hits        27732    27859     +127     
+ Misses       6107     5985     -122     

Impacted Files	Coverage Δ
src/mam/mod_mam.erl	97.40% <ø> (ø)
src/mam/mam_iq.erl	84.21% <100.00%> (+0.87%)	⬆️
src/mam/mod_mam_muc.erl	75.34% <100.00%> (+0.11%)	⬆️
src/mam/mod_mam_params.erl	100.00% <100.00%> (ø)
src/mam/mod_mam_pm.erl	89.55% <100.00%> (+0.04%)	⬆️
src/pubsub/mod_pubsub_db.erl	57.14% <0.00%> (-28.58%)	⬇️
src/ejabberd.erl	55.00% <0.00%> (-5.00%)	⬇️
src/inbox/mod_inbox_rdbms_async.erl	70.58% <0.00%> (-2.95%)	⬇️
src/mam/mod_mam_muc_rdbms_arch_async.erl	82.85% <0.00%> (-2.86%)	⬇️
src/ejabberd_s2s_out.erl	60.54% <0.00%> (-1.99%)	⬇️
... and 22 more

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[mongoose-im]

small_tests_24 / small_tests / 28cb7a1
Reports root / small

---

small_tests_25 / small_tests / 28cb7a1
Reports root / small

---

dynamic_domains_pgsql_mnesia_24 / pgsql_mnesia / 28cb7a1
Reports root/ big
OK: 3354 / Failed: 0 / User-skipped: 88 / Auto-skipped: 0

---

ldap_mnesia_24 / ldap_mnesia / 28cb7a1
Reports root/ big
OK: 1897 / Failed: 0 / User-skipped: 520 / Auto-skipped: 0

---

dynamic_domains_mysql_redis_25 / mysql_redis / 28cb7a1
Reports root/ big
OK: 3337 / Failed: 0 / User-skipped: 105 / Auto-skipped: 0

---

dynamic_domains_pgsql_mnesia_25 / pgsql_mnesia / 28cb7a1
Reports root/ big
OK: 3354 / Failed: 0 / User-skipped: 88 / Auto-skipped: 0

---

ldap_mnesia_25 / ldap_mnesia / 28cb7a1
Reports root/ big
OK: 1897 / Failed: 0 / User-skipped: 520 / Auto-skipped: 0

---

dynamic_domains_mssql_mnesia_25 / odbc_mssql_mnesia / 28cb7a1
Reports root/ big
OK: 3354 / Failed: 0 / User-skipped: 88 / Auto-skipped: 0

---

pgsql_mnesia_24 / pgsql_mnesia / 28cb7a1
Reports root/ big
OK: 3728 / Failed: 0 / User-skipped: 97 / Auto-skipped: 0

---

internal_mnesia_25 / internal_mnesia / 28cb7a1
Reports root/ big
OK: 2018 / Failed: 0 / User-skipped: 399 / Auto-skipped: 0

---

elasticsearch_and_cassandra_25 / elasticsearch_and_cassandra_mnesia / 28cb7a1
Reports root/ big
OK: 2316 / Failed: 0 / User-skipped: 394 / Auto-skipped: 0

---

pgsql_mnesia_25 / pgsql_mnesia / 28cb7a1
Reports root/ big
OK: 3728 / Failed: 0 / User-skipped: 97 / Auto-skipped: 0

---

mysql_redis_25 / mysql_redis / 28cb7a1
Reports root/ big
OK: 3723 / Failed: 0 / User-skipped: 102 / Auto-skipped: 0

---

riak_mnesia_24 / riak_mnesia / 28cb7a1
Reports root/ big
OK: 2176 / Failed: 0 / User-skipped: 387 / Auto-skipped: 0

---

mssql_mnesia_25 / odbc_mssql_mnesia / 28cb7a1
Reports root/ big
OK: 3728 / Failed: 0 / User-skipped: 97 / Auto-skipped: 0

[arcusfelis]

ok