Reduce waiting time for cover on CI

[arcusfelis]

This PR addresses "Slow Travis? Slow cover."

Proposed changes include:

• Add KEEP_COVER_RUNNING variable

"Executing Export cover data from MongooseIM nodes" step works 10 times faster now.

[codecov]

Codecov Report

Merging #1893 into master will increase coverage by 0.01%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master    #1893      +/-   ##
==========================================
+ Coverage   74.61%   74.62%   +0.01%     
==========================================
  Files         307      307              
  Lines       28134    28135       +1     
==========================================
+ Hits        20991    20996       +5     
+ Misses       7143     7139       -4

Impacted Files	Coverage Δ
src/mongoose_cover_helper.erl	8.69% <0%> (-0.4%)	⬇️
src/mod_mam_muc_odbc_async_pool_writer.erl	69.1% <0%> (-4.07%)	⬇️
src/mod_bosh.erl	93.27% <0%> (-1.69%)	⬇️
src/rdbms/mongoose_rdbms.erl	67.83% <0%> (-1.51%)	⬇️
src/mod_bosh_socket.erl	80.66% <0%> (-1%)	⬇️
src/mod_mam_odbc_prefs.erl	95.09% <0%> (-0.99%)	⬇️
src/mod_mam_utils.erl	82.37% <0%> (-0.68%)	⬇️
src/mod_mam.erl	90.27% <0%> (-0.47%)	⬇️
src/mod_muc_log.erl	77.69% <0%> (-0.26%)	⬇️
src/ejabberd_sm.erl	83.72% <0%> (+0.67%)	⬆️
... and 5 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 7876629...c3b8a75. Read the comment docs.