[NativeProcessLinux] Integrate MainLoop
This commit integrates MainLoop into NativeProcessLinux. By registering a SIGCHLD handler with
the llgs main loop, we can get rid of the special monitor thread in NPL, which saves as a lot of
thread ping-pong when responding to client requests (e.g. qThreadInfo processing time has been
reduced by about 40%). It also makes the code simpler, IMHO.
Reviewers: ovyalov, clayborg, tberghammer, chaoren
Differential Revision: http://reviews.llvm.org/D11150