Encryption spec

[robert3005]

There's still some extraneous comments in the flatbuffer that need answering

[robert3005]

~I think if we want to add ctr mode we benefit from additional storage place for IV somewhere next to segment spec so we can actually randomly seek without reading first few bytes~ this is irrelevant whole segment is already in memory

[gatesn]

this is irrelevant whole segment is already in memory

This isn't necessarily true for memory-mapped files

[robert3005]

I think for memory mapped files the overhead of reading additional 12-16 bytes might not be that relevant but could would let us skip loading one extra page

[gatesn]

Closing for now, we can implement this later on request