tsan: use a single background thread for memory profiler and memory flush (and later for symbolizer flush)



git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@177627 91177308-0d34-0410-b5e6-96231b3b80d8
tsan: add a comment about magic numbers


git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@177628 91177308-0d34-0410-b5e6-96231b3b80d8