[core] Introduce SequencedScheduler and ParallelScheduler

This commit refactors `utils::ThreadPool` into a template
`ThreadedScheduler` class and provides aux type aliases.
So that it is possible to obtain a sequenced schedule,where
all the scheduled tasks are guarantied to be executed
consequently.

The sequenced lightweight scheduler is required by both the
orchestration thread and the refactored `FileSource` implementation.

1 files changed, 1 insertions, 1 deletions

diff --git a/src/mbgl/actor/scheduler.cpp b/src/mbgl/actor/scheduler.cpp
index cb0c7728ec..5fd9a133bd 100644
--- a/src/mbgl/actor/scheduler.cpp
+++ b/src/mbgl/actor/scheduler.cpp
@@ -28,7 +28,7 @@ std::shared_ptr<Scheduler> Scheduler::GetBackground() {
     std::shared_ptr<Scheduler> scheduler = weak.lock();
 
     if (!scheduler) {
-        weak = scheduler = std::make_shared<ThreadPool>(4);
+        weak = scheduler = std::make_shared<ThreadPool>();
     }
 
     return scheduler;