Add backpressure control and allow asynchronous flushing #153

Job	Run time
Build Unity package	0s
Build Libraries / Build Native library (win-x64)	8m 20s
Build Libraries / Build Native library (android-arm64)	1m 2s
Build Libraries / Build Native library (android-x64)	1m 4s
Build Libraries / Build Native library (android-arm)	1m 18s
Build Libraries / Build Native library (ios-x64)	35s
Build Libraries / Build Native library (ios-arm64)	41s
Build Libraries / Build Native library (linux-x64)	3m 9s
Build Libraries / Build Native library (osx-arm64)	7m 15s
Build Libraries / Build Native library (osx-x64)	6m 47s
Build Libraries / Build Native library (win-arm64)	3m 21s
Build Libraries / Build Native library (win-arm64-uwp)	3m 8s
Build Libraries / Build and pack .NET library	1m 14s
Build Libraries / Update pre-built libraries for Unity	0s
	37m 54s

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