fix #6033 and #6044

- trap errors inside timer callbacks
- protect timers from GC
- ignore uv alloc requests when handle->data==NULL
formatting fixes
Merge branch 'master' of https://github.com/JuliaLang/julia into jb/uvfixes
tolerate multiple calls to stop_timer
Merge pull request #6049 from JuliaLang/jb/uvfixes

fix #6033 and #6044