Reworked parallel executor to not block (#437)

Reworked parallel executor to not block

Author: aclysma

crates/bevy_app/src/task_pool_options.rs

@@ -52,7 +52,7 @@ impl Default for DefaultTaskPoolOptions {
     fn default() -> Self {
         DefaultTaskPoolOptions {
             // By default, use however many cores are available on the system
-            min_total_threads: 4, // TODO(#408): set `min_total_threads` back to `1`
+            min_total_threads: 1,
             max_total_threads: std::usize::MAX,
 
             // Use 25% of cores for IO, at least 1, no more than 4

crates/bevy_ecs/Cargo.toml

@@ -18,7 +18,7 @@ bevy_hecs = { path = "hecs", features = ["macros", "serialize"], version = "0.1"
 bevy_tasks = { path = "../bevy_tasks", version = "0.1" }
 bevy_utils = { path = "../bevy_utils", version = "0.1" }
 rand = "0.7.2"
-crossbeam-channel = "0.4.2"
 fixedbitset = "0.3.0"
 downcast-rs = "1.1.1"
 parking_lot = "0.10"
+log = { version = "0.4", features = ["release_max_level_info"] }