Don't set cluster_id if not needed

[arcusfelis]

This PR addresses "MIM-1396 Prepared query in mongoose_cluster_id causes errors on startup"

Proposed changes include:

• Don't insert, if not needed.
• Insert only when we generate a new ID.

Basically, we implement "select and just use it. If not found, create new and insert" logic.

It is a bit less robust:

• if a cluster got started without RDBMS, but after it got a new node with RDBMS, we would not get an update in DB.
• but this use case is pretty weird

There is still a race condition possible, when we try to insert, but someone has already inserted a ClusterID. we could handle it and retry, but it makes logic less clean. And I am pretty sure it's a rare case when two nodes go live for the first time. Could be avoided by adding retries, but nothing bad would happen anyway (we would see a warning in this case in logs, but that's it).

[codecov]

Codecov Report

Merging #3112 (6fb48f6) into master (9557370) will decrease coverage by 0.02%.
The diff coverage is 80.00%.

@@            Coverage Diff             @@
##           master    #3112      +/-   ##
==========================================
- Coverage   79.22%   79.19%   -0.03%     
==========================================
  Files         388      388              
  Lines       31858    31864       +6     
==========================================
- Hits        25238    25235       -3     
- Misses       6620     6629       +9     

Impacted Files	Coverage Δ
src/mongoose_cluster_id.erl	73.33% <80.00%> (-4.45%)	⬇️
src/ejabberd.erl	45.00% <0.00%> (-10.00%)	⬇️
..._distrib/mod_global_distrib_outgoing_conns_sup.erl	80.00% <0.00%> (-8.00%)	⬇️
src/cassandra/mongoose_cassandra_worker.erl	70.75% <0.00%> (-2.36%)	⬇️
src/global_distrib/mod_global_distrib_utils.erl	65.09% <0.00%> (-0.95%)	⬇️
src/mod_muc_log.erl	77.72% <0.00%> (ø)
src/ejabberd_c2s.erl	89.43% <0.00%> (+0.14%)	⬆️
...c/global_distrib/mod_global_distrib_server_mgr.erl	77.40% <0.00%> (+0.56%)	⬆️
...bal_distrib/mod_global_distrib_hosts_refresher.erl	73.58% <0.00%> (+1.88%)	⬆️
... and 1 more

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[chrzaszcz]

This looks cleaner than before, the only issue is with the statement from the description:

we would see a warning in this case in logs, but that's it

This isn't completely true: we will get an error and a warning because of execute_successfully. I logged the original issue because the error was logged always, now it can happen sometimes, but I think it's acceptable, I just want to let you know. The upsert would eliminate it.