Allow not copying the dictionary

[JeWe37]

For large dictionary files, the documentation recommends simply using mmap to open them. This works, however as internally open-vcdiff always copies this data into the VCDiffEngine immediately, the full file has to be present in memory, without being able to take advantage of the kernel's smart caching. This can easily lead to OOM for large files.

The change here is fairly minor, essentially it just allows to explicitly specify that the data does not need to be copied and to assume that its lifetime is sufficient.

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[JeWe37]

If anyone is even looking at this anymore, the memory requirements can be further reduced by increasing the kBlockSize and by decreasing the hash table size in this line where using a collision probability of 1/4 seems to work fine. This should probably be configurable dynamically.