Use Timeout<> in Process::RunThreadPlan

Summary:
Since the function is way too big already, I tried at least to factor out the
timeout computation stuff into a separate function. I've tried to make the new
code semantically equivalent, and it also makes sense when I look at it as a done
deal.

Reviewers: jingham

Subscribers: lldb-commits

Differential Revision: https://reviews.llvm.org/D27258

llvm-svn: 288326