Add profiler endpoint to HS2 and LLAP

Add a web endpoint for profiling based on async-profiler. This servlet should be added to httpserver so that HS2 and LLAP daemons can output flamegraphs when their /prof endpoint is hit. Since this will be based on https://github.com/jvm-profiling-tools/async-profiler heap allocation, lock contentions, HW counters etc. will also be supported in addition to cpu profiling. In most cases the profiling overhead is pretty low and is safe to run on production. More analysis on CPU and memory overhead here https://github.com/jvm-profiling-tools/async-profiler/issues/14 and https://github.com/jvm-profiling-tools/async-profiler/issues/131 

For the impatient, here is the usage doc and the sample output https://github.com/prasanthj/nightswatch/blob/master/README.md