Multi GPU partition fixes

[gswirski]

This simplifies partition calculations, fixes the wrong buffer size for merging partitions, and provides accessor methods for GPU to determine correct partition sizes.

min_partition_size is replaced by hash_rate - how many base elements are consumed by one hash iteration. This allows us to calculate at which point it makes sense to start a new partition. min_partition_size = hash_rate / E::EXTENSION_DEGREE.

Supersedes #339

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[irakliyk]

Looks good! Thank you! I left a few questions/comments inline.

[gswirski]

All comments have been addressed. Partition sizes are expressed in E elements, not base elements, I've added docs to clarify. Also, since it's taking so many attempts to get right, I've also added unit tests with the edge cases you've mentioned.

[irakliyk]

All looks good! Thank you!