Merge JournalManager, FlushManager and WriteBuffer into one FlushTracker #572

Job	Run time
cross (aarch64-unknown-linux-gnu)	2m 49s
examples (stable, ubuntu-latest)	5m 53s
cross (aarch64-unknown-linux-musl)	3m 43s
test (stable, ubuntu-latest)	3m 0s
examples (stable, windows-latest)	10m 11s
cross (i686-unknown-linux-gnu)	1m 57s
test (stable, windows-latest)	7m 7s
examples (stable, macos-latest)	6m 48s
cross (powerpc64-unknown-linux-gnu)	2m 54s
test (stable, macos-latest)	2m 35s
examples (1.74.0, ubuntu-latest)	5m 56s
cross (riscv64gc-unknown-linux-gnu)	2m 47s
test (1.74.0, ubuntu-latest)	3m 23s
examples (1.74.0, windows-latest)	9m 51s
test (1.74.0, windows-latest)	6m 15s
examples (1.74.0, macos-latest)	7m 22s
test (1.74.0, macos-latest)	2m 46s
	1h 25m 17s

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