Refactor async logic in bssl_shim slightly.

Move the state to TestState rather than passing pointers to them everywhere.
Also move SSL_read and SSL_write retry loops into helper functions so they
aren't repeated everywhere. This also makes the SSL_write calls all
consistently account for partial writes.

Change-Id: I9bc083a03da6a77ab2fc03c29d4028435fc02620
Reviewed-on: https://boringssl-review.googlesource.com/4214
Reviewed-by: Adam Langley <agl@google.com>