Enable TCMalloc background processing.

Check if tcmalloc::MallocExtension::NeedsProcessBackgroundActions returns true. If true, start a background thread to call  tcmalloc::MallocExtension::ProcessBackgroundActions. This function will not return and does its only sleeping.

This only applies to tcmalloc-google. On tcmalloc-gperftools, there is no thread started.

Whether to start a thread will controlled by a boolean startup parameter server parameter 
tcmallocEnableBackgroundThread .