Add prepared queries for the remaining parts of MAM - user & prefs

[arcusfelis]

This PR addresses "Add prepared queries for the remaining parts of MAM"

Proposed changes include:

• mod_mam_rdbms_user
• mod_mam_rdbms_prefs

[codecov]

Codecov Report

Merging #2977 (33a5ea5) into mu-prepared-queries (0b736f6) will increase coverage by 0.03%.
The diff coverage is 83.01%.

@@                   Coverage Diff                   @@
##           mu-prepared-queries    #2977      +/-   ##
=======================================================
+ Coverage                79.10%   79.13%   +0.03%     
=======================================================
  Files                      377      377              
  Lines                    32846    32823      -23     
=======================================================
- Hits                     25982    25974       -8     
+ Misses                    6864     6849      -15     

Impacted Files	Coverage Δ
src/rdbms/mongoose_rdbms.erl	64.28% <46.15%> (-1.12%)	⬇️
src/mam/mod_mam_rdbms_user.erl	72.58% <84.61%> (+8.19%)	⬆️
src/mam/mod_mam_rdbms_prefs.erl	97.67% <100.00%> (+6.68%)	⬆️
src/rdbms/rdbms_queries_mssql.erl	66.66% <0.00%> (-33.34%)	⬇️
src/elasticsearch/mongoose_elasticsearch.erl	76.92% <0.00%> (-7.70%)	⬇️
src/mam/mod_mam_elasticsearch_arch.erl	86.60% <0.00%> (-1.79%)	⬇️
src/wpool/mongoose_wpool.erl	81.00% <0.00%> (-1.00%)	⬇️
src/mod_muc_room.erl	77.54% <0.00%> (-0.06%)	⬇️
src/mod_muc_log.erl	77.29% <0.00%> (ø)
src/mod_roster.erl	79.71% <0.00%> (+0.23%)	⬆️
... and 2 more

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[arcusfelis]

https://circleci-mim-results.s3.eu-central-1.amazonaws.com/PR/2977/40312/elasticsearch_and_cassandra_mnesia.23.0.3-1/big/ct_run.test@default-2b0f9a12-81b8-46de-9bd3-9e4b40a77f88.2020-12-14_13.29.07/big_tests.tests.mam_SUITE.logs/run.2020-12-14_13.34.14/mam_suite.init_per_group.5426.html

Badarg inserting new prepared queries.

• Why do we enable rdbms module on ES and cassandra? Maybe the ETS table does not exist.
• Do we properly handle a case when a module is restarted? (two statements could be inserted in parallel too).

[chrzaszcz]

A few questions from me...

[chrzaszcz]

Great!