Move setting up task_scheduler_init from executables to library

Make entry points of individual pipeline stages responsible for
configuring the task scheduler with requested number of threads
passed in corresponding configuration bundle (ie. follow extractor).

src/tools/contract.cpp

@@ -187,8 +187,6 @@ int main(int argc, char *argv[]) try
     util::Log() << "Input file: " << contractor_config.base_path.string() << ".osrm";
     util::Log() << "Threads: " << contractor_config.requested_num_threads;
 
-    tbb::task_scheduler_init init(contractor_config.requested_num_threads);
-
     osrm::contract(contractor_config);
 
     util::DumpSTXXLStats();