perf: Load expansion bundles asynchronously in the background (#345)

* perf: Load expansion bundles asynchronously in the background

This commit completely overrides ExpansionLoader so that asset bundles
are loaded in async and in parallel while part compilation is being
done. These take a _really_ long time to decompress so it isn't able to
completely hide the latency but it seems to improve overall loading time
by ~10s on a stock + KSPCF install. As the time taken in part
compilation increases the time saved should continue to improve until
part compilation takes longer than about 13s, which is how long it takes
to unload the `serenity_assets` asset bundle.

The implementation details are:
- we now start our own LoadExpansionsV2 coroutine immediately after
  patching which waits one frame then starts async background loads
  of all the asset bundles
- once the bundles are loaded the coroutine waits until StartLoad is
  called and then goes and does the normal load for each of the
  expansion bundles in the usual order.
- I have also moved checksum calculations off to a background thread,
  though I doubt this makes much of a difference.

I haven't tried to optimize anything else other than moving the asset
bundle load off the main thread because I don't think there's much worth
optimizing there.

Other compatibility notes:
- this patch disables itself if the OptionalMakingHistoryDLCFeatures
  patch is enabled and configured to skip loading of the expansions.

* Update CHANGELOG with new performance patch

---------

Co-authored-by: JonnyOThan <jonnyothan@gmail.com>

Author: Phantomical

CHANGELOG.md

@@ -2,6 +2,7 @@
 
 ##### Unreleased
 **New/improved patches**
+- New performance patch: [**ExpansionBundlePreload**](https://github.com/KSPModdingLibs/KSPCommunityFixes/pull/345) : Loads DLC assetbundles asynchronously, saving several seconds during loading if DLC is installed. Thanks @Phantomical!
 - **MinorPerfTweaks** [now includes a patch to make PQS loading faster](https://github.com/KSPModdingLibs/KSPCommunityFixes/pull/343) on scene switches, saving ~1 second when switching from KSC to a vessel.  Thanks @Phantomical!
 - **MinorPerfTweaks** [now includes a patch to optimize refreshing part context windows](https://github.com/KSPModdingLibs/KSPCommunityFixes/pull/338). Thanks @Phantomical!
 

GameData/KSPCommunityFixes/Settings.cfg

@@ -492,6 +492,9 @@ KSP_COMMUNITY_FIXES
   // Improve the responsiveness of the part list when switching between categories, sorting and searching by tag.
   FasterEditorPartList = true
 
+  // Load the expansion bundles in the background while performing part compilation.
+  ExpansionBundlePreload = true
+
   // ##########################
   // Modding
   // ##########################

KSPCommunityFixes/KSPCommunityFixes.csproj

@@ -164,6 +164,7 @@
     <Compile Include="Modding\ModUpgradePipeline.cs" />
     <Compile Include="Performance\CraftBrowserOptimisations.cs" />
     <Compile Include="Modding\BaseFieldListUseFieldHost.cs" />
+    <Compile Include="Performance\ExpansionBundlePreload.cs" />
     <Compile Include="Performance\FasterEditorPartList.cs" />
     <Compile Include="Performance\ForceSyncSceneSwitch.cs" />
     <Compile Include="Performance\AsteroidAndCometDrillCache.cs" />