[event] fix ading user triggered events

[wgtdkp]

This PR has two small fixes

1. Add a timeout value for the async call event so that the event loop will not exit because of no events.
2. User triggered event struct (with fd equals -1) should not listen on READ, WRITE events.

[codecov-commenter]

Codecov Report

Merging #128 into master will increase coverage by 0.00%. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #128   +/-   ##
=======================================
  Coverage   62.65%   62.66%           
=======================================
  Files          52       52           
  Lines        4753     4754    +1     
=======================================
+ Hits         2978     2979    +1     
  Misses       1775     1775           

Impacted Files	Coverage Δ
src/library/commissioner_safe.cpp	62.50% <100.00%> (+0.15%)	:arrow_up:
src/library/joiner_session.cpp	86.84% <100.00%> (ø)