Yay compatibility and predictability!

Pending:

- Tag BoringSSL repo.
- Tag gRPC repo.
- Build the plugin, zip it, and add it to the release.
- Push Cocoapods to trunk.

This reverts commit d88e15ce.

files

This reverts commit 48d833a9, reversing
changes made to 66e9d8e8.

As opposed to a flat directory, we now have the following structure:

- security
  -context
  - credentials
    - composite
    - fake
    - google_default
    - iam
    - jwt
    - oauth2
    - plugin
    - ssl
  - transport
  - util

We have not refactored the test code yet but this PR is already large
enough...

For some definition of "smart"... client_channel simply passes along
pollset/pollset_set, removing the need to instantiate a pollset_set in
the subchannel_call_holder: it's now up to the LB policies to handle the
pollset/pollset_set.