Reduce stringprepping in mod_offline

[NelsonVides]

get_max_user_messages gets its parameters from the #offline_msg record,
which when created took the jid:to_lus/1 from a full jid, hence its
arguments are already stringprepped.

resend_offline_message_packet will forward the server parameter down the
stack up to the timestamp function, which needs the stringprepped
parameter, but this was stringprepped already before, so this processed
version can be passed instead.

[mongoose-im]

9026.1 / Erlang 23.0.3 / small_tests / dcfc896
Reports root / small

---

9026.2 / Erlang 23.0.3 / internal_mnesia / dcfc896
Reports root/ big
OK: 1503 / Failed: 0 / User-skipped: 161 / Auto-skipped: 0

---

9026.4 / Erlang 23.0.3 / mysql_redis / dcfc896
Reports root/ big
OK: 2766 / Failed: 0 / User-skipped: 234 / Auto-skipped: 0

---

9026.3 / Erlang 23.0.3 / odbc_mssql_mnesia / dcfc896
Reports root/ big
OK: 2771 / Failed: 0 / User-skipped: 229 / Auto-skipped: 0

---

9026.7 / Erlang 23.0.3 / elasticsearch_and_cassandra_mnesia / dcfc896
Reports root/ big
OK: 331 / Failed: 0 / User-skipped: 38 / Auto-skipped: 0

---

9026.6 / Erlang 23.0.3 / ldap_mnesia / dcfc896
Reports root/ big
OK: 1405 / Failed: 0 / User-skipped: 259 / Auto-skipped: 0

---

9026.5 / Erlang 23.0.3 / riak_mnesia / dcfc896
Reports root/ big
OK: 1629 / Failed: 0 / User-skipped: 181 / Auto-skipped: 0

---

9026.9 / Erlang 22.3 / pgsql_mnesia / dcfc896
Reports root/ big / small
OK: 2784 / Failed: 0 / User-skipped: 216 / Auto-skipped: 0

[codecov]

Codecov Report

Merging #3009 (b639c66) into master (bd13278) will decrease coverage by 0.00%.
The diff coverage is 94.11%.

@@            Coverage Diff             @@
##           master    #3009      +/-   ##
==========================================
- Coverage   79.15%   79.14%   -0.01%     
==========================================
  Files         377      377              
  Lines       32763    32751      -12     
==========================================
- Hits        25933    25921      -12     
  Misses       6830     6830              

Impacted Files	Coverage Δ
src/ejabberd_c2s.erl	89.33% <ø> (ø)
src/mongoose_hooks.erl	92.24% <ø> (ø)
src/offline/mod_offline_mnesia.erl	51.47% <50.00%> (+1.47%)	⬆️
src/offline/mod_offline.erl	77.83% <100.00%> (-0.24%)	⬇️
src/offline/mod_offline_chatmarkers.erl	91.66% <100.00%> (-0.14%)	⬇️
src/offline/mod_offline_rdbms.erl	83.82% <100.00%> (-0.90%)	⬇️
src/offline/mod_offline_riak.erl	90.66% <100.00%> (-0.36%)	⬇️
src/cassandra/mongoose_cassandra_worker.erl	73.11% <0.00%> (-1.89%)	⬇️
src/wpool/mongoose_wpool.erl	81.00% <0.00%> (-1.00%)	⬇️
src/muc_light/mod_muc_light_db_mnesia.erl	85.00% <0.00%> (-1.00%)	⬇️
... and 8 more

---

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[chrzaszcz]

Great work!