Use HostType variable in mod_mam_rdbms_prefs/user

[arcusfelis]

This PR addresses MIM-1412 (kinda)

Proposed changes include:

• prefs/user and utils modules use HostType variable and host_type type where needed.
• no functional changes.
• ah, and I've refactored hooks there.

[codecov]

Codecov Report

Merging #3157 (5ff96d7) into master (0d82dc9) will decrease coverage by 0.05%.
The diff coverage is 93.75%.

@@            Coverage Diff             @@
##           master    #3157      +/-   ##
==========================================
- Coverage   79.89%   79.84%   -0.06%     
==========================================
  Files         396      396              
  Lines       32335    32274      -61     
==========================================
- Hits        25834    25768      -66     
- Misses       6501     6506       +5     

Impacted Files	Coverage Δ
src/gen_mod.erl	73.61% <ø> (ø)
src/mam/mod_mam.erl	88.88% <ø> (ø)
src/mam/mod_mam_rdbms_user.erl	80.55% <77.77%> (+7.97%)	⬆️
src/mam/mod_mam_cache_user.erl	87.71% <100.00%> (-2.70%)	⬇️
src/mam/mod_mam_rdbms_prefs.erl	97.01% <100.00%> (-0.66%)	⬇️
src/mam/mod_mam_utils.erl	89.89% <100.00%> (ø)
src/elasticsearch/mongoose_elasticsearch.erl	76.92% <0.00%> (-7.70%)	⬇️
src/mam/mod_mam_rdbms_async_pool_writer.erl	62.96% <0.00%> (-4.63%)	⬇️
src/mongoose_tcp_listener.erl	76.59% <0.00%> (-4.26%)	⬇️
...bal_distrib/mod_global_distrib_hosts_refresher.erl	73.58% <0.00%> (-3.78%)	⬇️
... and 8 more

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[chrzaszcz]

Looks good, I only have one minor style-related remark. Fixing is optional.