Don't explicitly kill greenlets in unit tests.

We don't actually need to do this.
Fail more informatively when tests are affected by syncback race condition.

If you have a syncback service running in the background and talking to the
test database, it will grab actions enqueued by the tests, causing the tests to
fail cryptically. Detect this case, and raise a more informative error.
Remove unused test fixture.
Revert encryption of secrets, blocks.

Existing data on s3 has to be encrypted before this is rolled out
so as to preserve data integrity and ensure accurate encryption status
is stored in the Block table.

Commits reverted -
a8f4be: "Check for None in migration 088."
c9bd77: "Helper script to re-encrypt secrets."
af6cb8: "New scheme for EAS passwords encryption."
eb47f4: "[SCHEMA][DEPENDENCIES] Encryption of secrets (passwords, refresh_tokens) and blocks."
        Conflicts:
	        inbox/models/roles.py
	        requirements.txt
	        tests/data/base_dump.sql
	        tests/general/test_storage.py

Migrations renamed, test DB updated too.

Sync runs as before.
All tests pass.
Revert "Fail more informatively when tests are affected by syncback race condition."

This reverts commit 2495171ea09ac48f08b8e1bf82b17041acb5fa5f.

Reason for rollback: causes IOError when you try to run the whole test suite.
Remove tests/general/test_storage.py too
Migration for the encryption revert.