Chain DB: audit for repeatedly opening the same file for reading a block

While we typically use ImmutableDB iterators to efficiently stream blocks in the ChainDB, sometimes we read blocks one-by-one, in particular when we're reading from the VolatileDB, as it doesn't provide iterator-like functionality. For example, when copying blocks from the VolatileDB to the ImmutableDB, the blocks from the VolatileDB are read one-by-one, which requires opening, seeking, reading, and closing a file for each block.

* Change interface to allow keeping file handles open (VolatileDB + ChainDB)
* Discourage single-shot reads by renaming the function, e.g., `s/getBlock/getBlockInefficiently/`
* Provide the equivalent of `pread` in the FS layer?