OpenCrypto: Convert sessions to pointers instead of integers

[cem]

Summary

Track session objects in the framework, and pass pointers between the
framework (OCF) and drivers. Avoid lots of redundancy and complexity in
individual drivers.

Inspired by
https://lists.freebsd.org/pipermail/freebsd-arch/2018-January/018835.html .

Test Plan

Proof of concept conversion to pointer sessions owned by OCF, instead of individual drivers.

I haven't converted the bulk of the drivers yet, and these just compile — have not tested yet. Looking for feedback on the general approach before I go bulk converting.

[cem]

Rebase over r336269