Make the block db open RAII style by calling it in the BlockManager
constructor.
Before this change the block tree db was needlessly re-opened during
startup when loading a completed snapshot. Improve this by letting the
block manager open it on construction. This also simplifies the test
code a bit.
The change was initially motivated to make it easier for users of the
kernel library to instantiate a BlockManager that may be used to read
data from disk without loading the block index into a cache.
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